Technically, an engine misfire is the result of incomplete combustion (or zero combustion) inside one or more of an engine’s cylinders. But to you, the driver, the problem will usually feel like hesitation or shaking when the car is running. On modern vehicles, the check engine light will also pop on when there’s a misfire. An engine misfire is when one or more cylinders do not produce power, and there are several possible causes, ranging from a malfunctioning spark plug to a blocked fuel injector or faulty oxygen sensor.

When the check engine light illuminates, your car’s primary computer, which is often referred to as the powertrain control module (PCM), will store a diagnostic trouble code (DTC) in its memory. Codes P0300 to P0312 are the primary DTCs associated with an engine misfire.

Why Do Engines misfire?

There are many reasons your engine might be misfiring. It can be a sensor that can cause the engine to misfire or there can be other causes. It is important to get it diagnosed and repaired as soon as possible to prevent damaging other components.

Why is the problem happening? Unfortunately, there are many potential causes, so answering that question isn’t always easy. If your car is suffering from a misfire, it’s best to let a professional figure out why.

The following are possible causes of misfiring:

  • Ignition system problems

At the mention of the term “misfire,” most people think of worn-out spark plugs, but many do not realize that the spark plugs are just one part of the ignition system. A typical modern ignition system contains a variety of components, including the control module, crankshaft position sensor, coil packs, wiring and, of course, the spark plugs. Issues with any of these parts can result in an engine misfire.

  • Air and fuel delivery problems

Air and fuel mix together inside the engine, then the mixture is ignited by the spark plug, the explosion sets the engine in motion, creating the rotational force needed to propel your car down the road. Any issue that stops the air and fuel mixture, which could either be a failed fuel injector or a vacuum leak, will automatically make the engine misfire.

  • Emissions equipment problems

Late model cars have an array of emissions equipment that help to minimize the amount of pollution released into the atmosphere. A couple of examples include the exhaust gas recirculation (EGR) system and the positive crankcase ventilation (PCV) system.  In some cases, issues with emissions equipment can alter the engine’s air and fuel mixture enough to create a misfire. 

  • Engine mechanical problems

Many people also don’t realize that an engine mechanical problem can cause a misfire, each cylinder inside the engine contains a piston that must compress the air/fuel mixture for complete combustion. And when the piston is moving upward, the cylinder must remain completely sealed off to create adequate compression. Internal engine problems can prevent the cylinder from sealing properly, leading to a loss of compression and an engine misfire.

  •  Sensor and module problems

Today’s vehicles contain a plethora of sensors, many of which the PCM uses to determine control of critical functions, such as fuel delivery and spark timing. A such, sensor problems can easily contribute to an engine misfire.  Although relatively rare, a problem with the PCM itself can also cause a misfire.

  • Control circuit problems

All the input and output engine management devices (i.e., sensors, ignition coil packs, etc.) are connected where needed via electrical circuits. Problems within these circuits, such as damaged wiring or a loose connection, can cause an engine misfire.