Fault code P0303 – a step-by-step guide to diagnosing and fixing it

P0303 indicates that the third cylinder of the engine is misfiring, meaning that the combustion cycle was incomplete. It is usually a sign of fuel pressure or compression problems, as well as faults in the ignition system or delivery system. The car's engine management system stores the DTC when it detects fluctuations in the rotational speed of the engine crankshaft.

• A blown or carbon fouled spark plug. Fouling is often an indicator of a rich air-fuel mixture (black or grey deposits), oil contamination (black, shiny), or coolant leaks (ashy, white).
• A clogged or malfunctioning fuel injector. Clogging is normally caused by a contaminated nozzle and the formation of hard carbon deposits inside the component.
• A faulty mass air flow (MAF) or O2 sensor in the car
• An out of sync camshaft and crankshaft or position sensor malfunctions
• Low fuel pressure or a broken fuel pump. This is often caused by contaminated fuel or electrical faults.
• Reduced compression in cylinder #3. This can occur if the pistons or piston rings are damaged, the head gasket has blown, valves are leaking, or the timing belt needs replacing.
• Leaks in the air intake, exhaust, and vapour recovery systems, etc. Air leakages may affect the air-fuel ratio.
• Damage to the high-voltage wires or burnt ignition coils. The coils may burn out due to excessive wear or other causes of overheating.
• A broken crankcase ventilation hose or faulty EGR valve in the car
• A worn distributor rotor

• A poor fuel economy
• Decreased engine power
• Sudden stalling when idling
• Activation of the check engine light
• Jerking or hesitation when accelerating
• A strong fuel smell
• Difficulty starting the vehicle and fluctuating engine speeds

• For a more accurate diagnosis and to quickly find the cause, scan for DTCs using an OBD2 tool. Read through and decipher the related codes. For example, “P0087” may indicate that the pressure in the fuel system is too low, or “P0337” or “P0340” may indicate a problem with the ignition coil circuit.
• Firstly, you will need to inspect the main ignition components. Check the tips of the spark plugs for signs of damage or significant carbon fouling. It is also important to check the high-voltage wires and connectors for signs of arcing, cracks, loose connections, thermal damage, and corrosion. The coils or coil packs should also be examined. Replace the components if necessary.
• Make sure that the distributor cap is in good condition and seals properly as water contamination or dirt could damage the rotor. You may need to install a new cap and rotor.
• Try to repair leaks in the air intake and exhaust systems. This may require you to replace damaged seals, pipes or cylinder head gaskets. You must also ensure that the clamps are tightened properly. However, some leaks may require professional welding services.
• Check whether the sensors are working properly using a multimeter or oscilloscope. The readings should be compared with the standard values specified by the manufacturer in the vehicle’s repair manual. Install new devices or sensor wires if necessary.
• Replace worn valve train components and adjust the tension of the timing belt or chain.
• Clean or “flush” the fuel injectors to remove any blockages or carbon build-ups. This can be done using special cleaning additives. You can also test the system’s fuel pressure by installing a pressure gauge at the fuel rail. Make sure to follow the instructions closely as some injection systems operate with extremely high pressure levels.
• Restore the compression in the cylinder by installing new parts, such as piston rings, cylinder valves, or the head gasket.