Dissecting the mystery of the misfire code

Jan. 1, 2020
Knowledge is power in this business. As a young technician, I couldn't wait to learn something new I could use to become more proficient, and make more money in the process. This is information you can give to newer technicians or those who feel less

Much has been written about how to find and repair misfire codes, but diagnosis can be even easier if technicians understand what the codes are and how they are set.

Knowledge is power in this business. As a young technician, I couldn't wait to learn something new I could use to become more proficient, and make more money in the process. This is information you can give to newer technicians or those who feel less than competent when it comes to driveability diagnosis.

Misfire codes are one of the most common reasons for a check engine light to turn on. And I know many technicians who think the way to solve this problem is by tuning a vehicle up. Let's take a look first at exactly what a misfire code is, and how it is set to put a quick end to that overly simplistic statement.

With the advent of on-board diagnostics II (OBDII), misfire monitors were needed to maintain a vehicle's emissions quality. Typically, manufacturers detect the presence of misfires by monitoring the signal from the crankshaft position sensor (CKP). Variances in the rpm of this signal could indicate that a cylinder didn't add its share of power to the engine, temporarily slowing things down. If this continues past a specific number of engine revolutions, a misfire code is generated. Severe, catalyst-damaging misfires are generally referred to as "Type A" misfires, while those that result only in unacceptable increases in emissions are referred to as "Type B" misfires.

Time to ask your tech customer to put his thinking cap on: What does this brief description of the monitor's strategy reveal?

The first thing that comes to mind is the fact that a misfire code can be set by any condition that affects the cylinder's ability to provide its equal share of power to the engine. That could be low compression, poor airflow through the cylinder, improper air/fuel mixture or a weak ignition system. The second thing is because the Engine Control Module (ECM) is relying on the CKP, its signal integrity must be good and manufacturing variances in the sensor's reluctor wheel must first be learned by the ECM.

And the third thing that comes to mind is the fact that, because misfire monitors are sometimes intentionally ignored by the ECM when crank speed might be influenced by operating conditions, such as driving on a rough road, it is possible to have a misfire that can be felt without a code being set.

Finding the offending cylinders is getting easier. Most aftermarket scan tools with enhanced data will allow a tech to access misfire counters — historical, current (active) or both. Ford now has misfire PIDs (Parameter Identifications) to help provide more accurate information on the conditions the vehicle was under when the misfire(s) occurred, in addition to their Mode $06 misfire information.

Vehicles that employ Control Area Network (CAN) protocol will also list these individual cylinder results in their Mode $06. For more details on Ford's Mode $06 misfire tests, see November 2006's Motor Age or visit www.autoservicetech.com.

Once a tech has the cylinders identified, he or she should take a look at the freeze frame information. Keep in mind that the freeze frame data is stored at the time the code is stored and may actually be the vehicle condition some time after the misfire first started. Misfire detection strategies measure the number of misfires over a given number of engine revolutions.

For most manufacturers, misfires are monitored over a 200-revolution counter and a 1,000-revolution counter, in addition to being monitored during initial start-up. The number of misfires detected that would trigger a fault depends on the operating conditions and the emissions regulations in effect when the vehicle was made.

Typically, misfires that would result in emissions levels exceeding that limit would log a Type B misfire, while a misfire level that would lead to overheating of the catalytic converter (with resulting damage) would log a Type A misfire. A Type B misfire is a two-trip code, requiring the same fault to be seen during the next monitor run before turning on the Malfunction Indicator Lamp (MIL), while a Type A is a one-trip code that will cause the MIL to "flash" its warning to the driver.

In any case, a specified number of misfires must be detected before the ECM will consider it a problem, and they will show up in the misfire history for each cylinder.

Fuel trim factor

Have the technician take a look at the fuel trims stored. Generally, if the total fuel trims are above normal but less than +18 percent, start the diagnosis with the ignition system. If it's greater than +18 percent, the tech may want to take a look at the engine's mechanical condition and fuel system first.

If the technician sees a code P0316 (misfire detected on the first 1,000 revolutions after startup), then something is "dead." Suspect an engine mechanical fault first, then have the tech move on to a "no fuel" or "no spark" inspection.

A code P0300 (random cylinder misfire detected) tells the tech that the allowable misfire count has been exceeded, but not by any one particular cylinder. Look for intermittent faults, weak ignition systems or intake leaks or leaking exhaust gas recirculation (EGR) systems as possible causes.

For the more capable technicians, there is a PC-based program that will help identify missing cylinders, as well as offer ideas on possible causes from information gathered from the oxygen sensor signal.

Vacuum-pressure transducers can be used to analyze exhaust gas pulses at the tailpipe. Another program is incorporated into a PC-based lab scope and allows cylinder power balance testing as well.

In any case, caution the tech to avoid "tunnel vision" and remember that misfire codes can be caused by a wide variety of culprits. The first challenge is to identify the affected cylinder(s), then think about what the offending cylinders have in common.

Is the problem on only one bank? Is it limited to cylinders that are next to each other? Do the cylinders share an ignition coil? Is the misfire intermittent or repeatable? Does it occur only at idle, at highway speed or across the board? How was the vehicle being operated when the code was stored? Can you duplicate the problem? Are there any modifications?

Misfire codes may be common, but the causes of misfires are varied and multiple. If a tech can verify the integrity (or weakness) of the major sub systems at the beginning of a diagnosis, he or she will nail misfire faults easier and faster.

Peter Meier is an ASE, CMAT, a member of iATN and a full-time technician with CarMax in Tampa, Fla.