Error code P0300 is easily one of the most infamous error codes in auto maintenance as it indicates random misfire problem in your engine. The worst thing about it is, it can mean different things, and could be caused by different problems. This entry will teach you how to figure them out to have your car running in good condition in no time.


Error code P0300 means that the car’s computer has identified a problem in engine’s cylinders – at least one of the cylinders is not firing properly.


The error code P0300 points out random or multiple misfires. Most modern vehicles come with a software strategy built into its computer, which is designed to detect engine misfire. The computer can even detect the specific cylinder that is not firing properly. Engine misfire simply means lack of combustion, which is usually caused by low compression, lack of spark, bad fuel quality or metering, or unmetered air getting to the engine. Other less obvious causes includes uncommanded EGR (Exhaust Gas Recirculation) flow. Many times, the Check Engine light will also individuate the specific engine number that misfired through a last digit code. For example, if it is the first cylinder that misfired, then the error code reading will be P0301; second cylinder will be P0301, and so on.

Common Symptoms

Common symptoms of this error code include:

  • Check Engine Light is on
  • Engine stumble and/or hesitate
  • It is harder to start the engine
  • Lack or loss of power

Possible Causes

  • Defective computer and sensor issues, fuel injector or related wiring problems
  • Poor fuel quality or running out of gas frequently
  • EVAP (Evaporative Emission System) concerns such as fuel vapor leaking to the engine
  • Wrong fuel pressure
  • EGR problems; EGR valve leak or restricted ports
  • Problems on base engine, such as low compression, timing issues or valve train problems
  • Ignition system problems, such as faulty spark plugs, bad coil condition or faulty wiring, ignition module issue or related wiring, faults on ignition related sensor and wiring issue.
  • Faulty coil, oxygen sensor(s), fuel injector(s), and/or catalytic converter(s)
  • Burned exhaust valve
  • Faulty camshaft position sensor

How to check

There are many reasons  that can result to engine misfire, and they may or may not be detected by the computer and trigger the vehicle’s Check Engine light.

You can start by inspecting for proper fuel and ignition system operation. Then, you can head to less-likely reasons such as problems on EGR and EVAP systems.

To check, use the scan tool to read for misfire codes or any diagnostic troubleshooting codes. Misfire will not always set diagnostic trouble code, so it is important to conduct a cylinder balance test to pin point which of the cylinder(s) is misfiring.

Once you have detected the problematic cylinder(s), check whether it’s getting spark.

Remove and check the spark plug wire (or COP ignition coil and Boot) and spark plug if you see signs of wear and tear, anti-freeze, carbon tracks, etc.

If the problem is not on the ignition system, then check the fuel injectors, vacuum leaks, compression tests, etc.

How to fix

Double Check the Check Engine Light

First thing you need to do is find out which cylinder is misfiring. You can do this by using an error code reader and take note of the last digit of the code.

You can also pinpoint which cylinder is misfiring by using an infrared thermo gun and test the exhaust temperature of each cylinder.

Check the Exhaust Temperature

Start the engine while it’s still cold and then quickly take a reading in front of every exhaust port on the manifold of the cylinders while keeping the same placement of beam on each individual port. Problematic cylinders will be usually colder than the others that are working properly. For example, most ports will record 190° while the other records 81°.

Remove the Fuel Injector Wire

If you still can’t find clear results, then start the engine and remove electrical connector of fuel injector of each cylinder (one at a time please) and observe the performance of the engine. You can observe changes on the engine’s performance when detaching the misfiring cylinder.

Remove Ignition Coil Connector

You can also get the same result by removing the connector for the coil wire.

Check Connectors and Wirings

If the engine stumbles or hesitates, then check all the connectors and wiring that lead to your cylinders (such as spark plugs, spark plug wires, distributor cap, and rotor). If the component has been there for quite some time, then consider replacing them as part of your regular maintenance. You can also check the coils (and coil packs).

Check the Catalytic Converter

In some cases, the problem can be found in the catalytic converter. If you can smell rotten egg odor from the exhaust, then it’s a sign your converter needs to be replaced.

Parting Tips

It is always good to have the factory service manual and an advanced scan tool to diagnose the random misfire effectively.

For more information about different OBD II error codes, browse through our website. If you need professional assistance, feel free to talk to Contact Us.