P0204 — Injector Circuit Open — Cylinder 4

What Is P0204?

P0204 is stored when the ECU detects an open circuit on the fuel injector for cylinder 4. An open circuit means the electrical path between the ECU and the injector is broken — the injector receives no signal and delivers no fuel to cylinder 4.

The effect is a complete misfire on cylinder 4, causing significant rough running, power loss, and unburnt fuel passing into the exhaust. This can damage the catalytic converter rapidly. Do not ignore or drive long distances with this code active.

Common Symptoms

Severe misfire on cylinder 4 — rough engine
Engine management light on, possibly flashing
Significant loss of power
Strong smell of unburnt fuel from exhaust
Engine vibration through chassis and steering
Increased fuel consumption
Potential catalytic converter damage if driven far

Common Causes

Failed injector solenoid — open circuit winding on cylinder 4 injector

Broken wire in the cylinder 4 injector wiring harness

Damaged connector pin at the cylinder 4 injector

Blown fuse or relay in the injector circuit — check the fuse box

Open circuit in the wiring loom between ECU and cylinder 4 injector

ECU driver fault — rare; suspect only after eliminating all other causes

How to Diagnose P0204

Test Injector Resistance

Disconnect the cylinder 4 injector connector and measure resistance across the injector terminals. A healthy petrol injector reads 12–16 ohms; diesel 0.5–2 ohms. Open circuit (infinite resistance) = failed solenoid — replace the injector.

Check the Connector

Inspect the connector for bent, corroded, or heat-damaged pins. A poor connection causes exactly the same open-circuit reading as a failed injector. Clean and reseat firmly before condemning the injector.

Test Wiring Continuity

Using a multimeter in continuity mode, test each wire from the injector connector back to the ECU. No continuity indicates a broken wire. Inspect the harness routing near the exhaust manifold — chafing and heat damage are common here.

Listen for Injector Click

With the engine running, use a stethoscope or long screwdriver to listen at each injector. A firing injector produces a rapid click. Silence on cylinder 4 confirms the injector is not operating.

Swap Injectors to Confirm

Swap the cylinder 4 injector with one from a different cylinder. If the misfire moves to the other cylinder, the injector is faulty. If it stays on cylinder 4, the wiring or ECU is at fault.

Replace Injector

Once confirmed faulty, replace the cylinder 4 injector. On diesel engines, programme the injector trim code into the ECU using specialist software to restore correct fuel delivery.

Catalytic Converter WarningA cylinder 4 misfire passes unburnt fuel into the catalytic converter, which overheats and can be permanently destroyed within minutes at high engine speed. Fix this fault urgently — do not drive long distances with P0204 active.

Verdict

Test injector resistance first. Open circuit means replace the injector. If resistance is correct, trace the wiring. Swapping injectors between cylinders is the quickest way to confirm fault location.