Fault code P0304 – a step-by-step guide to diagnosing and fixing it

P0303 is triggered when a misfire is detected in the engine cylinder #4, meaning that the combustion cycle was incomplete. This DTC is stored when the ECU module in the car identifies a ≥2% increase or decrease in the rotational speed of the car crankshaft. Misfiring is often caused by low pressure levels, a loss of compression, timing problems, or spark plug malfunctions.

• A faulty or dirty spark plug
• A worn ignition distributor rotor
• Damage to the high-voltage wires or coils
• A stretched timing chain or belt, worn gears or lobes, or asynchrony between the camshaft and crankshaft
• A faulty CKP, CMP, MAF or oxygen sensor
• Low fuel pressure or a broken fuel pump
• Clogged or defective fuel injectors
• Leaks in the air intake, exhaust or gas recirculation system
• A damaged crankcase ventilation hose or bad EGR valve
• A leaking head gasket

• A poor fuel economy
• Decreased engine power
• Sudden stalling when idling
• Activation of the check engine light
• Jerking or hesitation during acceleration
• A strong fuel smell
• Difficulty starting the engine
• Fluctuating RPM during acceleration

• To find the cause quickly, scan the control unit using an OBD2 diagnostic tool. Read through and decipher the related DTCs. For example, “P0087” indicates a low fuel pressure, and “P0337” or “P0340” indicate a problem with the ignition coil circuit.
• Inspect and repair the main components of the ignition system. Check the tips of the spark plugs on a car for carbon or oily deposits, as well as signs of burning or electrode damage. You should also check the high-voltage wires and connectors for arcing, cracks, loose connections, thermal damage, and corrosion. Next, examine the ignition coils or coil packs. Replace any damaged or broken components.
• Check whether the distributor cap is still reliable as water contamination or dirt could cause the rotor to corrode or wear away. You may need to install a new cap and rotor.
• Repair any vacuum leaks. This may require you to replace damaged seals, pipes or cylinder head gaskets. Tighten loose clamps to ensure that the intake and exhaust systems are securely sealed. Some leaks may require professional welding services.
• Test whether the sensors are working properly using a multimeter or oscilloscope. Compare the test results with the correct 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 injection system to unblock the components. This can be done using a special injector cleaner. You can test the delivery system’s fuel pressure by installing a pressure gauge at the fuel rail. Pay attention to the safety instructions for this procedure as injection systems can operate with extremely high pressure levels, posing a safety risk.
• Restore the compression in the cylinder by installing new parts, such as piston rings, cylinder valves, or the head gasket.