Injector Circuit/Open Cylinder 4
The P0204 code indicates that the Powertrain Control Module (PCM) has detected a malfunction or open circuit in the fuel injector circuit specifically for cylinder number 4. The PCM sets this code when it monitors the voltage or resistance of the injector circuit and finds it falls outside the manufacturer's expected operating range.
This code should be addressed promptly because driving with a disabled cylinder causes poor performance and engine imbalance. Prolonged driving can also lead to catalytic converter damage or further engine issues due to the associated misfire.
Common symptoms
- Illuminated Check Engine Light
- Engine misfire resulting in a rough idle
- Noticeable lack of engine power during acceleration
- Decreased fuel efficiency
- Engine hesitation or stalling
Likely causes
- Faulty fuel injector on cylinder 4
- Damaged, frayed, or broken wiring in the cylinder 4 injector circuit
- Loose, corroded, or poor electrical connection at the cylinder 4 injector harness
- Open or shorted injector control circuit in the wiring harness
- Failing Powertrain Control Module (PCM) or internal injector driver (rare)
What to check before replacing parts
Inspect the cylinder 4 injector wiring and connector
Look for: Look for visible damage, frayed wires, melted insulation, or corrosion and loose pins inside the electrical connector.
Listen to the fuel injector operation
Look for: Use a mechanic's stethoscope or a long screwdriver to listen to the cylinder 4 injector while the engine is running. A lack of a rhythmic clicking sound indicates the injector is not firing.
Test the fuel injector resistance
Look for: Disconnect the injector and use a multimeter to measure its internal resistance across the terminals. An infinite reading (open) or a reading outside of factory specifications indicates a faulty injector.
Check the injector harness with a noid light
Look for: Plug a noid light into the cylinder 4 injector harness connector and crank the engine. If the light flashes rhythmically, the PCM and wiring are delivering the correct signal; if it does not flash, there is a wiring or PCM issue.
Perform a continuity and voltage test on the circuit
Look for: Use a multimeter to verify that the injector is receiving constant battery voltage on the supply wire and check the ground/control wire for continuity back to the PCM.
This page explains the generic P0204 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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