P0300 OBD-II Trouble Code: Random/Multiple Cylinder Misfire Detected

P0300 code definition

Random/multiple cylinder misfire detected

What the P0300 code means

Most vehicles will have 4-6 cylinders. Each cylinder will fire a spark plug, one after another, in a smooth and continuous manner. The spark plug fires a spark, using the flammable fuel air mixture, and sets fire to the fuel. The fuel then releases energy that powers the crankshaft, which must continuously turn smoothly as the car is moving.

If more than one cylinder misfires, this will increase or decrease the revolutions per minute (RPM) of the crankshaft. If that increase and/or decrease is more than 2 percent, the Powertrain Control Module will store the P0300 trouble code.

If the percentage of the increase or decrease in revolutions per minute (RPM) is between 2 percent and 10 percent, the Check Engine Light will come on. If the percentage of the increase or decrease in revolutions per minute (RPM) is greater than 10 percent, the Check Engine Light will blink on and off, as to alert the driver of a more serious misfiring problem. The P0300 trouble code indicates that there is a misfire in random or multiple cylinders.

What causes the P0300 code?

• Spark plugs that are damaged or worn
• Spark plug wires and/or coils that are damaged or worn
• Distributor cap that is damaged or worn (only if applicable to the vehicle)
• Rotor button that is damaged or worn (only if applicable to the vehicle)
• Fuel injectors that are defective
• EGR valves or tubes that are clogged
• Ignition timing that is off
• Vaccum leaks
• Fuel pressure that is low
• Head gasket(s) that are leaking
• Distributor cap that is cracked
• Camshaft sensor that is faulty
• Crankshaft sensor that is faulty
• Mass air flow sensor that is faulty
• Oxygen sensor that is faulty
• Throttle position sensor that is faulty
• Catalytic converter that is faulty
• PCM is faulty

What are the symptoms of the P0300 code?

• Check Engine Light that is on or flashing
• Car may take longer to start up, or does not start up at all
• Car dies while stopped
• Car idles rough
• Car may hesitate while accelerating
• Lack of power while driving
• Increase of fuel consumption

How does a mechanic diagnose the P0300 code?

• Uses OBD-II scanner to retrieve freeze frame data and all of the trouble codes that have been stored by the Primary Control Module (PCM)

• Completes a test drive of the vehicle to see if the P0300 trouble code comes back

• Reviews live data to identify which cylinders are showing misfire data

• Inspects spark plug wires or coil packs for wiring that is damaged or worn

• Inspects spark plugs for excessive wear, cracks, or breaks

• Inspects coil pack wiring at the harness and connector, for breaks, or corrosion

• Replaces spark plugs, spark plug wires, coil packs, coil pack wiring at harness and connector, as necessary

• If the P0300 trouble code is still present after the spark plugs, spark plug wires and/or coil packs have been replaced, they will inspect the fuel injectors and fuel injector system for faults

• Older vehicles have a distributor cap and rotor button system. For these vehicles, the mechanic will inspect the distributor cap and rotor button for damage, cracks, or excessive wear

• If other related trouble codes have been stored by the PCM, the mechanic will need to diagnose and repair these issues as necessary. Test drive the vehicle again to see if the P0300 trouble code comes back.

• If the P0300 trouble code returns, the compression system should be checked. A fault in the compressor is rare but possible for this code.

• If the P0300 trouble code still persists, there could be a problem with the PCM. This may require replacing or reprogramming. This is also rare.

Common mistakes when diagnosing the P0300 code

One common mistake is ruling out the possibility of a faulty cylinder, fuel injector, or PCM. It is also common that the diagnosis and repair of other related trouble codes, is not completed.

Other related trouble codes, a faulty cylinder, a faulty fuel injector(s), and/or a faulty PCM can all cause the misfiring problem.

How serious is the P0300 code?

The P0300 trouble code is considered serious. The reason for this is the resulting driveability problems that are present when this trouble code is stored in the PCM. The car may not start and/or cause hazardous conditions during the operation of the vehicle. For these reasons, the P0300 trouble code requires immediate attention.

What repairs can fix the P0300 code?

• Replacing damaged spark plugs
• Replacing damaged or worn spark plug wires and/or coils
  
• Repairing or replacing clogged EGR valves and/or tubes
• Repairing vacuum leaks
• Repairing or replacing leaking head gaskets
• Replacing faulty camshaft sensor
• Replacing faulty crankshaft sensor
• Replacing faulty mass air flow sensor
• Replacing faulty oxygen sensor
• Replacing faulty throttle position sensor
• Replacing faulty fuel injectors
• Replacing faulty catalytic converter
• Diagnosing and repairing other related trouble codes
• Replacing distributor cap and rotor button, as well as wires, coils, and plugs, (only if applicable to the vehicle)
• If necessary, repair fault internal engine components.
• Replacing engine (if cylinder damage exists)
• Replacing a faulty PCM

Additional comments for consideration regarding the P0300 code

Do not assume that the spark plugs and wires are responsible for the P0300 trouble code. There are other components of the system that could cause this error code that should be diagnosed and inspected prior to repair.

Also, there are other related trouble codes that could cause the P0300. After repairs, if the P0300 trouble code returns, inspect and diagnose these other trouble codes if applicable. Always test drive the vehicle to see if the P0300 trouble code returns after making any repairs.

Make sure the spark plugs are gapped properly and with precision. To gap the spark plugs properly, it will be necessary to use a feeler gauge.

If the spark plug gap is not precise, this will cause a misfire.The vehicle manufacturer provides the spark plug gap specifications. These specifications can be found on a sticker under the hood of the vehicle or obtained from any local part store.

Need help with a P0300 code?

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