Fault code P0300 – a step-by-step guide to diagnosing and fixing it

P0300 means that misfires have been detected in one or more engine cylinders. It may be accompanied by similar DTCs such as P0302, P0303 and P0304. In these cases, the last number of the code corresponds to the number of the cylinder that is misfiring.

Misfires will increase or reduce the RPM. If there is an increase or decrease of more than 2%, the control unit will trigger the DTC.

• Faulty or worn car spark plugs or wires. For example, the tips of the plugs can melt, wires are affected by thermal and mechanical damage, and the power supply could be disrupted by a broken or short circuit.
• Damaged or defective car ignition coils
• Problems with the fuel supply caused by low pressure, faulty or clogged injectors, a blocked filter, or bad car fuel pump.
• Problems with the crankshaft, camshaft, oxygen, throttle position and MAF sensors.
• A faulty EGR valve or leaking head gasket
• Vacuum leaks
• Incorrect engine timing, a worn out or slipping timing belt and jammed valves

• The engine is difficult to start or keeps stalling
• A strong fuel smell coming from the exhaust
• The engine runs rough or is shaky
• Jerking during acceleration
• A loss of engine power
• Increased fuel consumption
• The “Check Engine” light is on

Repairing your vehicle will require a comprehensive diagnosis due to the long list of possible causes for this fault. You can start by doing a diagnostic scan using an OBD2 scan tool. Connect the tool to the vehicle via the data link connector and wait for it to check the system for errors. You should check all of the DTCs as this will help you to identify the source of the problem quicker.

Then you will need to inspect the spark plugs and high-voltage wires for signs of damage or contamination. Check whether the electrical components are working properly using a multimeter or an oscilloscope. Replace the plugs if necessary and then check again to see whether the car’s engine still misfires.

You may also need to check the fuel pressure levels, inspect the vacuum lines for leaks, check the compression in the cylinders and or adjust the tension of the timing belt. Defective parts, including clogged fuel injectors, should be promptly repaired or replaced.