What Is Engine Code P0300?

  • P0300 Technical Definition: 
  • P0300 Meaning: Random Engine Misfires
  • Most common cause: Can be caused by any fuel, emission or ignition component
  • Risks for the engine/driver: MEDIUM You should get it checked out ASAP to prevent possible engine damage
  • Emissions severity: HIGH. The car probably won’t pass emissions testing
  • Estimated repair time: Depends on exact fault
  • Estimated repair cost: Difficult to estimate

The engine code P0300 is stored in memory when the ECU detects a misfire in the engine.

The ECU is constantly monitoring the information of all its sensors in order to detect any misfire. During each cylinder combustion cycle, the ECU tries to sense any problem and set the appropriate data trouble code.

However, under certain conditions, the ECU is unable to isolate the misfire origin or it detects random misfires changing from one cylinder to another without a logic pattern. The P0300 code is a way to alert about engine misfires, even when it does not provide any additional information.


7 Common Symptoms When Code P0300 is Present

  1. Check Engine Light Lit
  2. Difficulty to start the engine
  3. Decreased engine performance
  4. Poor fuel economy
  5. Increased emissions
  6. Random engine misfires
  7. Engine hesitation during idle

Possible Causes For Error Code P0300

The most common causes of data trouble code P0300 are:

  • Oxygen sensor wiring (open, shorted, burnt) 
  • Oxygen sensor connector (loosely, corroded, disconnected or bent pins)
  • Camshaft position sensor wiring (open, shorted, burnt) 
  • Camshaft position sensor connector (loosely, corroded, disconnected or bent pins)
  • Crankshaft position sensor wiring (open, shorted, burnt) 
  • Crankshaft position sensor connector (loosely, corroded, disconnected or bent pins)
  • Vacuum leaks.
  • Deficient Fuel Pressure System
  • Bad or clogged Mass Air Flow sensor (MAF)
  • EGR system malfunction
  • Bad ignition coil, spark plug wires or spark plugs
  • Low engine compression
  • Bad upstream oxygen sensor
  • Bad Cam position sensor
  • Bad Crankshaft position sensor
  • Deficient catalytic converter
  • Bad exhaust valve

How To Diagnose The Engine Code P0300

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

IMPORTANT: the diagnostic procedures listed below assume you only have the data trouble code P0300 present. If you also have any other code related to the ignition system, emissions system or fuel system then you should start diagnosing them first.

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.

  • Read data trouble codes and take note of them.
  • Clear data trouble codes memory.
  • Perform a driving cycle (at least 5-10 minutes).

In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

> Engine compression tests: before starting a complete diagnostic process is necessary to check engine condition. An engine with low compression may cause a code P0300 (besides many other problems) and would need immediate attention. Please refer to our Oil On Spark Plug article for detailed instructions on how to diagnose engine compression.

2. Visual Inspection

> Vacuum hoses: perform a meticulous visual inspection of all vacuum lines, including MAP sensor, PCV, brakes vacuum booster, fuel pressure regulator, etc. Also, check the air intake hose looking for any sign of deterioration or leak.

> MAF condition: to inspect the MAF condition you will need to remove it from the vehicle first. Please refer to our MAF article for detailed instructions on how to diagnose and clean this important sensor.

> Oxygen Sensor wiring and connector: perform a meticulous visual inspection of the upstream O2 sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

> Exhaust Gas Recirculation (EGR) inspection: any leak in the exhaust, or in the EGR system may cause an erratic misfire condition. Please refer to our EGR article for more information regarding EGR diagnosis.

> Catalytic Converter inspection: any leak in the exhaust, or in the CAT system may cause an erratic misfire condition. Please refer to our Catalytic Converter article for more information regarding CAT diagnosis.

> Ignition Coil(s) inspection: perform a meticulous visual inspection of the ignition coil(s) wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug each coil and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

> Camshaft position sensor inspection: perform a meticulous visual inspection of the CMP wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the CMP and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

> Crankshaft position sensor inspection: perform a meticulous visual inspection of the CKP wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the CKP and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

Fix any wiring problem before continuing.

3. Fuel System Tests

> Fuel Pump pressure: using the appropriate adapter connect a fuel pressure gauge to the engine test port and check the fuel pressure (engine off and engine running). Compare the values with the OEM literature. A lean condition may occur if the fuel pressure is not within its normal values. A low fuel pressure will cause a deficient fuel delivery and as a consequence a possible code P0300.

> Other Tests: A clogged fuel filter or even extremely dirty fuel injectors could also cause a low fuel pressure condition. Check the in-line fuel filter and the tank fuel filter element. Replace them as necessary. Checking the fuel injectors condition is difficult without a proper test bench. 

4. Scan Tool Tests

> Mass Air Flow Sensor: Using a professional scan tool can save you significant time regarding MAF diagnostics. As mentioned earlier, refer to our MAF article for detailed instructions on how to diagnose this sensor.

> Upstream Oxygen Sensor Test: the upstream oxygen sensor is a key component in the emissions control system. You will need to verify its wiring, connector, electrical resistance as well as scan tool output. You can find further instructions on how to perform an exhaustive diagnostic of this sensor in our oxygen sensor article.

> Ignition Coil(s) test: using a professional scan tool can save you significant time regarding ignition coil(s) diagnostics. Please refer to our bad ignition coil article for detailed information regarding diagnosis.


How To Repair Error Code P0300

Depending on the diagnostics results you may need to do the following:

  • Repairing the oxygen sensor(s) wiring and/or connectors.
  • Repairing the CMP wiring and/or connectors.
  • Repairing the CKP wiring and/or connectors.
  • Repairing the ignition coil(s) wiring and/or connectors.
  • Replacing the MAF sensor, CKP sensor, CMP sensor
  • Replacing the ignition coil(s)
  • Replacing the EGR
  • Repairing the exhaust pipe
  • Repairing vacuum leaks
  • Replacing the fuel pump, inline filter or other fuel system elements.
  • Replacing the upstream oxygen sensors.

As you can see, repairing code P0300 involves a lot of testing. It can be triggered by many different parts of the engine, sometimes more than one at a time. It's probably a good idea to leave this one to the professionals.