Attacking no-code misfires (2023)

This article has been updated from its original 2018 version.

In most cases today’s engine management systems allow for a quick and accurate diagnosis when driveability issues are present. When symptoms exist and there are no codes, it becomes a little more difficult. And when there are no codes and the issue is intermittent, the difficulty factor grows exponentially.

In this Tool Briefing we will discuss a multiple-symptom issue (with no codes), and which tools you can use to determine the cause and correct the issue.

Step 1 – Obtain vehicle specific information

Whenever you are faced with any problem with a vehicle, you need to obtain as much understanding of the issue, and the potential causes, that you can. When dealing with an intermittent problem, acquiring as much specific information as possible will not only allow you to speed the diagnostic process up, but be more accurate in your diagnosis.

One of the best sources of specific vehicle information is the driver of the vehicle. This may not necessarily be the person who brought the vehicle in. It could be a spouse, child, or even a co-worker that you need to speak with, in order to get the details of what the conditions are when the issue occurred/occurs.

Some critical questions that will help to duplicate the issue include:

  1. What speeds and load are present when it happens?
  2. Is the engine warmed up or cold?
  3. How much fuel is in the tank?
  4. What type of fuel do you use?
  5. What type of driving when it occurs: highway or stop-and-go?
  6. Have any repairs or services been performed prior to the symptom?
  7. Have any repairs been done to try to correct the issue?

These are just a few pieces of information that will allow the technician to duplicate the issue, which is one of the most critical steps to make sure the vehicle is repaired properly. In my opinion, if a technician cannot duplicate the issue, he or she should not repair or replace anything, mainly because they cannot repeat the problem to see whether it is fixed.

A critical part of vehicle specific information includes details such as the VIN, mileage, and if any codes are present. Use your scan tool to check for current (as well as history) codes and monitor status. History codes can be an indicator of issues that the vehicle has but are not present at this moment. This is the very definition of “intermittent”. Monitors that are not complete may indicate that codes have been recently cleared.

In this case, I interviewed the customer and found they had been experiencing an intermittent rough idle, along with an occasional hesitation when accelerating from a stop, mostly after the vehicle was warm. They also stated they took the car to an auto parts store that advertises a “free diagnosis.” The parts store connected their “computer” to the car and told them they needed spark plugs and ignition coils, and maybe fuel injectors. The customer decided they needed a second opinion and brought the car into our shop.

No existing codes were found, but the scan did show that many of the monitors were not complete, indicating the codes had probably been cleared recently. When there are no codes, but there is a misfire that would typically trigger a code, use your vehicle information database to determine what the enabling criteria for setting a code is. In the case of a misfire a P0300 (random or multiple cylinder misfire), or specific cylinder P0301-P0306 should have triggered. We used Mitchell 1 to look up the information for setting a P0300. One of the conditions that is required to set the code is a fuel level over 11 percent. During our initial inspection, we noted that the fuel level was well under a quarter tank on the gauge, which is why there may not be a current code.

We also checked for TSBs and pattern failures. There was a TSB for carbon build-up and the approved cleaning method, and another TSB for the use of “Top Tier” gasoline.

One other item of note on the initial inspection was the dash was indicating an oil change was due, and a scan of the oil life monitor also indicated zero life. The oil level was full and the oil appeared somewhat clean, but there are only 28,000 miles on the vehicle.

At this point we had a few ideas of what was wrong with the vehicle, but we decided another discussion with the customer was in order before proceeding. The answers to additional questions would likely reveal our next steps in order to confirm our thoughts.

Since we noted the fuel level was low, it would be good to know if they usually keep the level low. Consistently driving the vehicle with low fuel levels can contaminate the fuel injectors and pump with sediment.

The customer revealed they do not usually fill the tank because they don’t drive it that much. They also stated they use 87 octane fuel from the least expensive source they can find. It should be noted that GM recommends mid-grade, 89 octane fuel for this vehicle. They also said the oil had not been changed for about a year.

Armed with as much information as we were able to obtain from the customer, an initial scan, TSBs, and additional data from Mitchell 1, we have a strong suspicion the problems this customer is experiencing are due to carbon buildup on the back side of the intake valves.

This is a Gasoline Direct Injection (GDI) engine; these engines have a propensity to build carbon on the valves, especially if the vehicle operates under many of the conditions, we have found on this one. Consistently driving short trips does not allow the engine to reach and sustain a temperature that will boil off moisture in the crankcase. That, combined with extended interval oil changes, will allow oil to accumulate on the back side of the intake valves, eventually turning it into carbon. When carbon builds up, airflow past the valves is disturbed, which may cause either over-or under-fueling of the cylinders, therefore a misfire. Keep in mind this car is eight years old with only 28,000 miles, which calculates to only 3,500 miles per year.

Step 2 – Test the vehicle

Again, it is never a good idea to just jump into repairs without having duplicated the issue, and performing various tests to complete a diagnosis. With no codes to start with, the next step is to drive the vehicle in order to duplicate the issue. After driving the vehicle for around 20 minutes, we were able to experience the issue, and confirm it only misfires, at idle.

A scan of the data stream showed a few interesting items. The bank 2 Oxygen sensor voltage was continually biased higher than bank 1 voltage. The scan tool also provides a PID for long-term fuel trim average, for both banks. As seen in the image, the average for bank 2 is -99, meaning the injectors are being commanded to reduce the amount of fuel being delivered to that bank.

Based on the scan readings, it appears that bank 2 has either an injector leaking fuel into a cylinder, or a carbon build-up is disrupting airflow into the cylinders.

With the tendency of GDI engines to carbon up intake valves, and either disrupt air flow, or cause valves to stick, it is much more likely that a carbon buildup is the issue rather than a faulty injector. Being that the issue is intermittent and seems to be related to warm, not cold temperature, it seems to be a reasonable assumption that it is not a sticking valve. Sticking valves will typically be an issue with a cold engine.

With the low mileage on the vehicle, the apparent poor maintenance, and the conditions the vehicle is operated in, it appears the most likely cause of the misfire is carbon buildup.

In order to help document our diagnosis, we removed the spark plugs to see how much, if any, carbon was on the plugs. We also used a borescope to check for carbon on the piston tops. Checking for carbon on the intake valves would require removing the intake plenum, which the customer did not want to do at this time.

As seen in the image, there was a substantial amount of carbon on the plugs, especially considering the low number of miles on the car. Interestingly, cylinders on bank 1 all showed less carbon than bank 2. This can usually be explained by where PCV or vent hoses are introduced into the intake plenum.

Step 3 – The repair

Fuel injection flushes that are added into the fuel will not work to remove carbon from the valves, although it can help with cleaning the piston tops and in some cases, the injectors. Using a chemical flush that is introduced into the intake manifold will typically work on engines that have relatively low mileage like this one. We connected our injection cleaning machine to the engine and flushed the system. Our system uses a three-part chemical: one that is a solvent that loosens carbon, another to wash the carbon out, and another that adds a cleaner to the fuel.

Step 4 – Confirm the repair

Since this was an intermittent issue we wanted to make sure the vehicle was completely repaired. We have scan readings that showed an issue, carbon on the spark plugs, and a drive test where we experienced the problem. After cleaning the system, we drove the vehicle under the same conditions we experienced the misfire, and we did not feel a miss. We removed the plugs after we drove the car and they showed less carbon than they did previously. The image from the borescope also showed less carbon on the piston tops. Current scan tool readings also showed oxygen sensor and fuel trim readings in the normal range. Don’t forget if you are going to monitor PIDs like fuel trims after a repair, if the codes were cleared, your road test will need to include the proper drive cycle conditions to complete the monitor for that system, which in this case included adding fuel to the vehicle.

Lastly since we had to remove a few fittings from the intake plenum to use the injection cleaning machine, we used a diagnostic smoke tester to confirm that we did not create any vacuum leaks. This test only takes a few moments, but can save an incredible amount of time to assure a repair has been done properly.

To sum it up, this diagnosis and repair was relatively straightforward, and it would have been very easy to just go straight to performing an injection cleaning service. It would have cured the customer’s issue. The problem with jumping to a repair, rather than performing a complete diagnosis, is that your assumptions could be wrong and you do not fix the vehicle. Performing a complete diagnosis with supporting documentation before and after the repair allows you to share your findings with the customer and be sure the repair is complete.


What can cause a misfire with no codes? ›

With the low mileage on the vehicle, the apparent poor maintenance, and the conditions the vehicle is operated in, it appears the most likely cause of the misfire is carbon buildup.

Will all misfires throw a code? ›

Yes, an engine misfire will usually show codes and bring on a check engine light. In some cases, a misfire may not cause a check engine light, but it will almost always produce codes that can be found through diagnostic testing.

How many misfires does it take to throw a code? ›

Normally the misfire counts should be zero or close to zero for every cylinder. The OBD II system will usually NOT set a misfire code until the actual misfire count exceeds about two percent for any given cylinder.

How can we solve the problem of misfire? ›

Use a spark plug socket to remove the plug to get a good look at it. The damage you see will help you determine the cause of the misfire. If the spark plug is just old, replacing it may solve the problem. Make sure to replace and properly gap new spark plugs.

What are 3 common possible causes of a misfire? ›

The most common causes of misfires are worn, improperly installed, and mishandled spark plugs, malfunctioning ignition coils, carbon tracking, faulty spark plug wires and vacuum leaks.

What sensors can cause misfire? ›

A clogged or failed exhaust gas recirculation or crankcase ventilation valve or faulty oxygen sensor can send the wrong signals to the computer and cause misfires.

Can an O2 sensor cause a misfire? ›

Will a bad O2 sensor cause rough idle and loss of engine power? You bet. Moreover, you may also notice poor acceleration, engine misfires, and even stalling. Bad oxygen sensors disrupt all kinds of essential engine functions, including engine timing, combustion intervals, and air-fuel ratio.

What sensor picks up a misfire? ›

The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. A single misfire will cause a subtle change in the speed of the crank.

What type of misfire is most severe? ›

The type "A" misfire is the most serious condition and indicates impending catalyst damage. If detected, the Malfunction Indicator Lamp (MIL) may flash once per second to alert the driver that immediate service is required.

Can a misfire destroy an engine? ›

If left untreated, a cylinder misfire can lead to significant engine damage. Worse, if you experience a bad misfire while driving, it could result in an accident. This is why it's important to treat engine misfires as soon as you detect them.

How long should you wait if you misfire? ›

Any time the hammer or firing pin falls and the gun fails to fire, a hangfire should be assumed. The firearm should not be opened or unloaded, but should be kept pointing in a safe direction for 60 seconds with a modern cartridge—and two minutes for a muzzleloader.

Can a bad O2 sensor cause multiple misfires? ›

Fuel related misfires can be caused by many different things such as low fuel pressure, faulty or dirty fuel injectors, a faulty O2 sensor, a dirty or failing mass air-flow sensor, a faulty or dirty idle air control valve or a vacuum or intake leak.

Will a clogged catalytic converter throw a P0300 code? ›

The catalytic converter may certainly cause the P0300 random misfire code to show up due to the inability for the exhaust to properly breathe, which can cause un-burnt exhaust gases to reignite inside the catalytic converter. A clogged or failing catalytic converter can cause the vehicle to have poor fuel economy.

Can a vacuum leak cause a misfire? ›

Unfortunately, air leaks sometimes occur in an engine. Such leaks, which go by the name of vacuum leaks, throw off the air-to-fuel ratio. Specifically, by allowing excess air into the system, vacuum leaks create a lean fuel scenario. As covered above, lean fuel can lead to misfires which in turn lead to rough idle.

Can SeaFoam fix a misfire? ›

SeaFoam will only remove varnish and deposits in your vehicle's fuel system. So it may or may not reduce a misfire if it is throttle body or fuel injection related. It is a good product and helps maintain one critical system for your vehicle, the fuel system.

Can a dirty intake cause misfire? ›

Engine misfires, rough idling and hard starts can all be traced to a clogged engine air filter. The dirty air filter restricts the air supply to the engine causing unburned fuel to form a soot residue that accumulates on the spark plug.

Can a bad coil pack cause misfire? ›

Usually, if a coil pack is bad, there will be a loss of fire or spark in one or more cylinders. This causes what's commonly referred to as misfiring. Misfiring can cause drag on the crankshaft, and usually results in a very poor performing engine.

Why are all my cylinders misfiring after a tune up? ›

There might be an issue with an ignition coil, fuel injector, or valve cover gasket. Sometimes the valve cover will leak oil into the spark plug hole. When there is enough oil, it can misfire. I would suggest bringing the vehicle to a shop that is more familiar with misfire diagnosis.

How do you tell if a coil pack is causing a misfire? ›

What Are the Signs of a Faulty Ignition Coil?
  1. Loss of Power. This is one of the first symptoms of ignition coil failure. ...
  2. Check Engine Light On. ...
  3. Poor Fuel Economy. ...
  4. Backfiring. ...
  5. Misfiring Engine. ...
  6. Hard Starts and Stalling. ...
  7. Spluttering and Coughing Sounds. ...
  8. Jerking and Vibrating.
Sep 3, 2021

Can a clogged catalytic converter cause misfire? ›

Yes, a bad catalytic converter is a likely cause of an engine to misfire. A blocked catalytic converter will typically overheat, which can cause damage to your car's engine and result in a misfire.

Can throttle sensor cause misfire? ›

If you start to experience engine misfires, stalling, or rough idling when the car is stopped, it can also be a warning sign of a failing TPS. You don't want to wait to get this checked out! If the idling appears off, it means the computer is unable to identify the fully shut throttle.

What are the symptoms of faulty engine sensors? ›

First, what are the signs an O2 sensor may have gone bad?
  • Check Engine Light Comes On.
  • Noticeable Loss of Fuel Efficiency.
  • Sulfur or 'Rotten Egg' smell from Exhaust.
  • Black smoke from exhaust.
  • Emission levels reach high levels.
  • Your engine hesitates, skips, begins bucking or has power surges.
Oct 2, 2019

Can oil pressure sensor cause misfire? ›

Low oil pressure has caused your engine to run out of fuel, dropping the RPMS, causing a misfire.

Can co2 sensor cause misfire? ›

A bad sensor can disrupt the way the engine runs which can lead to a feeling of roughness when idling or operating the car in general. Engine misfires. Because the O2 sensor dictates the lean or rich quality of the fuel that your engine receives, a broken O2 sensor can lead to your engine misfiring.

What code will a bad O2 sensor throw? ›

P0162: O2 sensor circuit malfunction (bank 2, sensor 3)

Will mass airflow sensor make it misfire? ›

If an oxygen sensor or mass airflow sensor is failing, it could give incorrect data to your engine's computer, causing the misfire. When a vacuum line is broken, it can cause a fuel-injected motor to misfire.

Can a crank position sensor cause misfire? ›

A faulty crankshaft sensor will cause your engine to misfire as incorrect fuel injection readings weigh in. When this problem becomes especially apparent, the engine may stall and have difficulty restarting. Rough idling and backfiring are two more indicators that something is wrong.

Can a camshaft position sensor cause a misfire? ›

Engine Misfires: A failing camshaft position sensor can cause your engine to misfire. Transmission Shifting Problems: The data sent to the engine control module can stop the transmission from shifting properly.

Why is my misfire worse at idle? ›

Generally, the cause of a misfire at idle is an incorrect air-fuel mixture. This can be caused by a faulty O2 sensor, a fuel injector that needs cleaning, or even vacuum leaks.

Will an obd2 scanner tell which cylinder is misfiring? ›

OBD II can give you an exact count of the misfires cylinder by cylinder (which you can find in Mode $06 with a scan tool), but it can't tell you what's causing the misfires. Every engine will experience an occasional misfire.

Can an engine misfire fix itself? ›

Misfires don't simply go away - they need to be addressed immediately. Even if they don't get worse, they certainly won't get any better unless you take the car to a mechanic.

Does a misfire waste more gas? ›

Bad Ignition System Parts

A misfire occurs when the fuel in an engine cylinder does not combust. Since unburnt fuel cannot power your vehicle, this ends up wasting gas and lowering your fuel economy.

Can you drive long distance with a misfire? ›

While you can still drive a car with a misfire, it's not a good idea. A misfire could be a sign of several different engine issues. If you continue to drive without resolving the issue that caused the misfire in the first place, it could potentially cause more extensive damage to your engine over time.

How much damage can a misfire do? ›

A misfiring cylinder can cause a proportional loss of power. For example, if one cylinder misfires in a four-cylinder engine, the car will lose 25 percent of its power.

Is a misfire the same as a hangfire? ›

Hang fires happen when the firing pin has struck the primer and there is a delay before the gun fires. This can occur for several reasons, such as a faulty firing pin or spring, defective primer, or other cartridge-related problems. A misfire is when the primer fails to ignite the powder.

Do misfires happen at idle? ›

A misfire will cause the engine to momentarily stumble, or lose rpms, and then regain its normal engine speed. The misfire will usually reappear, either under specific operating conditions or randomly. A misfire may occur when your engine is idling, causing a rough or uneven idle.

Will a bad spark plug throw a code? ›

bad spark plugs can cause your engine to misfire. the engine's computer uses sensors to detect these misfires and will create a code that turns on the check engine light. a flashing check engine light indicates the misfire is severe enough to cause damage to your catalytic converter.

Can a clogged air filter cause P0300? ›

This can be caused by a faulty engine computer, as well as by mechanical issues like a clogged air filter or faulty fuel pump.

Can bad ignition coils cause P0300? ›

A malfunctioning ignition, fuel, or internal engine failure can all lead to engine misfires. The most common cause of this is defective or worn-out spark plug coil packs, especially if you haven't had a service. These are some of the factors that the engine code P0300.

Can dirty fuel injectors cause P0300? ›

The fault codes usually associated with a clogged fuel injector can range from misfire codes to lean codes. Fault codes associated with a clogged fuel injector are the P0300 thru P0308 series, which indicates the engine controller is seeing an engine misfire.

What are the symptoms of a vacuum leak on intake manifold? ›

Four Signs Your Car Has a Vacuum Leak That Needs Fixing
  • Sporadic Idling. A vacuum leak introduces excess air into the engine, and this directly affects how your engine runs. ...
  • Engine Hesitation. Your engine trouble will not be limited to your idling if you have a leak. ...
  • Vacuum Sounds. ...
  • 4 Check Engine Warning.
Mar 1, 2021

What are signs of a bad vacuum leak? ›

Four Signs My Car's Engine Has a Vacuum Leak
  • Check Engine Light Warning. ...
  • Engine Hesitation or Stalls. ...
  • High or Sporadic Idling. ...
  • Hissing or Sucking Noises.
Sep 1, 2021

What causes a misfire in one cylinder? ›

Lean misfire causes that only affect one cylinder include a dirty fuel injector, an open or shorted fuel injector, or a problem in the fuel injector driver circuit (wiring or PCM). Compression problems that may cause a misfire include a burned exhaust valve, bent intake or exhaust valve, or leaky head gasket.

Can you have a bad coil pack and no check engine light? ›

If one ignition coil is inoperative, a check engine light will come on. There is however no such code with the description “ignition coil X is faulty”.

Can a vacuum leak cause a single cylinder misfire? ›

Vacuum leaks, especially those that are confined to one cylinder, will cause the engine to idle unevenly and possibly misfire. This is because the vacuum leak allows additional air to reach the affected cylinder, diluting its air/fuel mixture.

Can a bad MAF sensor cause misfire? ›

Even a dirty MAF sensor can cause a lean code and/or misfire to occur. The engine may be stalling because it isn't getting enough throttle opening.

Can a bad cam sensor cause misfire? ›

Engine Misfires: A failing camshaft position sensor can cause your engine to misfire. Transmission Shifting Problems: The data sent to the engine control module can stop the transmission from shifting properly.

Can a crankshaft sensor cause a misfire? ›

A faulty crankshaft sensor will cause your engine to misfire as incorrect fuel injection readings weigh in. When this problem becomes especially apparent, the engine may stall and have difficulty restarting. Rough idling and backfiring are two more indicators that something is wrong.

Can one bad fuel injector cause multiple misfires? ›

Good quality fuel injectors rarely cause misfires whereas, poor quality or faulty injectors cause destructive misfires. The vehicle can experience various misfires when a faulty injector is a reason; it can lead to a loss in acceleration, power, and reduction in fuel efficiency.

What can cause a multiple cylinder misfire? ›

A leaky head gasket will usually cause multiple cylinders to misfire, especially ones next to each other. If you move a few spark plugs around and the issue stays localized within certain cylinders instead of localized to problematic spark plugs, you should check your head gasket.

What code will a bad ignition coil throw? ›

With most modern vehicles, a faulty ignition coil is enough to turn on the Check Engine Light. If you have an OBD-II diagnostic scanner, you may see a P0351 ignition code.

How do you test an ignition coil for a misfire? ›

Use a spark tester to check the ignition coil.
  1. Plug the tester into the coil.
  2. Attach the ground wire.
  3. Plug in the coil connector.
  4. Adjust the spark gap to the correct measurement.
  5. Start the engine.
  6. If there's spark, great, it works! If there's no spark, it's a bad coil.
Aug 6, 2021

Can a cracked ignition coil cause a misfire? ›

A weak or damaged ignition coil can cause engine misfires, stalling, poor performance, and lower fuel economy. A completely faulty ignition coil can often keep the vehicle, especially a 4 cylinder vehicle, from running at all.

How do you test if a coil pack is good or bad? ›

To diagnose a coil pack, set the multimeter to the 200 Ohms range, place the positive and negative probes on the identical terminals of a coil, and check the multimeter for a reading. A value between 0.3 ohms and 1.0 ohms means the coil is good, depending on the model.

Should all coil packs be replaced if one fails? ›

So, why replace only one coil at a time? Replacing one coil might cure the misfire and codes for now, but the other coils could fall prey to the same failure and codes. Replacing all for the coils at a time will save a comeback. This is why some ignition coil manufacturers package coils in sets.


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