Diagnosing Engine Misfire Codes

Diagnosing Engine Misfire Codes

On OBD II vehicles from 1996 to present day, generic engine misfire codes can be set for a variety of reasons and produce codes in the P0300 range. Learn what causes these failures to register in the vehicles power train control module. Review symptoms and possible solutions for when an engine miss situation occurs. Discover how fixing this code as soon as possible can avoid more serious problems. In a study done back in 2010 the engine misfire unseated the gas cap code as the most common check engine light malfunction set in memory. Both of these codes remain extremely common problems for motorists today. This article will focus solely on malfunctions that will register a diagnostic trouble code.

How is an Engine Miss Detected?

The main component in detecting a single or multiple cylinders that are not firing correctly would be the crankshaft sensor. Modern crankshaft sensors are capable of not only detecting the exact position of the crankshaft but its acceleration rate. After each properly functioning cylinder is fired the crankshaft experiences a push increasing its rotational speed.

Using crankshaft acceleration data supplied from the sensor the power train control module is able to monitor the power output from each individual cylinder. When it detects abnormalities in this data range it can choose to flag a particular cylinder for problems or if multiple cylinders have issues it can set a code for random misfire. As an example of the numbering system for the set codes, a random misfire is P0300. Individual cylinders will set their own codes if the problems are isolated. Cylinder number three would register P0303 and cylinder number seven would register a P0307.

Is a Misfire Harmful to the Automobile?

The short answer is it can be. The problem with this condition is if an engine misfires, unburned fuel is expelled into the exhaust system making its way to the catalytic converter. This raw gas can ignite and melt the internal components that allow the catalytic converter to reduce tailpipe emissions. Therefore, if a misfire remains unresolved further damage can be done to other expensive components on the vehicle.

Since Manufacturers are responsible for maintaining emission related equipment and aware of the possible damage that can result, they have built in some features to reduce this collateral damage. They use the monitoring capabilities of the crankshaft sensor and the power train control module to measure the severity of the problem. If the results of this monitoring perceive the engine miss to be excessive the computer can shut off the fuel to the affected cylinders. In essence they stop the injector from opening which prevents unburned fuel from entering the catalytic converter.

Symptoms of a Misfire

The severity of how this condition feels to the driver depends on a few variable factors. The most important is if the condition is intermittent or a permanent failure where a completely dead cylinder is the result. If one cylinder stops functioning the size of the engine is another determining factor of how bad this feels to the driver. If you lose one cylinder on an V8 automobile you will still notice the problem, but on a four-cylinder engine that is now reduced to running on three cylinders the symptoms of lack of power, bucking and surging are much more noticeable.

What Causes a Misfire Code

Unfortunately, many different items can cause an engine miss. With that said some items or more common than others. In my own personal experience when diagnosing individual cylinders that have set a misfire code, ignition system problems seem to be more common than any other of the possibilities. Ignition malfunctions can be caused by worn or defective spark plugs, defective ignition coils or the secondary ignition wires that connect the coil to the spark plug. Problems on the fuel system side are also capable of preventing individual or random cylinders from firing. If an injector is clogged or not opening complete combustion will not take place on that cylinder.

Mechanical problems are also capable of stopping proper operation. If the injector works but the intake valve fails to open the cylinder is starved of combustible resources and will not fire correctly. Along the same lines of thinking, low or uneven compression can also cause a misfire condition. One of the reasons the engine misfire code is commonly found on today’s automobiles is because it can be caused by many different problems. A logical path of diagnosis, often through the process of elimination will lead you to the root cause. When this is determined remember that automobiles 1995 and newer are subject to federal emissions laws that make manufacturers responsible for repairs of specific items under warranty.

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