MG ZS (Gen2) P0304 Cylinder 4 Misfire Fault

The P0304 Cylinder 4 Misfire Fault indicates that the engine control module (ECM) has detected repeated combustion failures in cylinder 4. This fault affects the engine management, ignition, and fuel injection systems and is identified by monitoring crankshaft speed fluctuations that correspond to misfires occurring specifically in cylinder 4. It applies to the MG ZS Gen2 (ZS11 platform), introduced from 2024 onwards.

Symptoms

  • Check Engine Light illuminated
  • Flashing Check Engine Light during severe misfires
  • Rough or unstable idle
  • Noticeable engine vibration
  • Hesitation during acceleration
  • Reduced engine power
  • Poor fuel economy
  • Hard starting
  • Increased exhaust emissions

Possible Causes

  • Worn or faulty cylinder 4 spark plug
  • Faulty cylinder 4 ignition coil
  • Faulty cylinder 4 fuel injector
  • Low compression in cylinder 4
  • Vacuum leak affecting cylinder 4
  • Damaged wiring or connectors to the ignition coil or injector
  • Faulty crankshaft or camshaft position sensor
  • Valve train or timing-related fault
  • Engine control module (ECM) fault (rare)

Diagnosis

Diagnosis begins by confirming the stored fault code and reviewing freeze-frame data and cylinder-specific misfire counters using a compatible diagnostic scan tool. The cylinder 4 spark plug, ignition coil, fuel injector, and associated wiring should then be inspected and tested, while compression and leak-down testing may be required to rule out mechanical engine faults. Manufacturer diagnostic procedures may also include swapping ignition or fuel system components between cylinders and analysing live engine data to accurately identify the source of the misfire.

For New Zealand vehicles, using OEM diagnostic procedures helps technicians determine whether the fault originates from the ignition system, fuel delivery system, or an internal engine condition before replacing components.

Accurately diagnosing this fault often requires manufacturer testing procedures, wiring diagrams, connector pinouts, ignition system specifications, fuel injector test procedures, compression specifications, and system calibration information. Access to the factory service manual significantly reduces diagnostic time and helps prevent unnecessary parts replacement.

Applicable Years

2024, 2025, 2026, onwards

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