Why does cylinder 4 seem to fail more often?

It is usually down to position. On many transverse engines cylinder 4 sits near the hot exhaust and gearbox, so its wiring and connector see more heat and are disturbed less often, which can hide a developing fault.

Can P0204 appear with other injector codes?

It can. If a shared fuse, relay, or loom section fails you may see several injector codes at once. A single P0204 on its own points squarely at the cylinder 4 circuit.

Is a used injector a safe repair?

It can work for petrol engines if the donor is known-good. For diesels, used injectors are risky because wear affects fuelling and they still need coding, so new or properly reconditioned units are wiser.

Does P0204 clear itself once fixed?

The light may stay on until the code is cleared or the ECU completes a few good drive cycles. Clearing it with a scanner after the repair confirms whether the fault returns.

Could a poor earth cause P0204?

Yes. The injector relies on a clean switching earth through the ECU. A high-resistance earth or damaged ground can make the circuit read open even when the injector itself is fine.

How urgent is this repair?

Urgent. Beyond the catalyst risk, a heavy misfire can overload the engine mounts and, on some engines, wash oil from the bore on the dead cylinder. Fix it promptly.

P0204 - Injector Circuit Open - Cylinder 4

What Is P0204?

P0204 is stored when the ECU finds an open circuit on the cylinder 4 injector. With the circuit broken the injector receives no signal, stays closed, and cylinder 4 produces no power while the rest of the engine carries on.

On most transverse engines cylinder 4 sits at the gearbox end, where the loom and connector are harder to reach and more exposed to heat. The misfire it causes is severe, and the unburnt fuel reaching the exhaust makes prompt repair important to protect the catalyst.

Common Symptoms

Severe misfire and shaking, worst at idle
Flashing engine management light
Marked power loss
Fuel smell from the tailpipe
Poor economy as fuelling is compensated elsewhere
Vibration felt at the back of the engine bay

Common Causes

Open injector winding - the cylinder 4 injector coil has failed open, the usual cause.

Heat-damaged wiring - the harness near the gearbox end can suffer from exhaust heat, breaking a conductor.

Connector fault - a corroded or dislodged connector on the least accessible injector.

Broken pin or terminal - a pin pushed back or snapped in the cylinder 4 plug.

Power supply issue - a blown fuse or relay feeding the injector bank.

ECU driver failure - loss of the cylinder 4 drive output inside the ECU, uncommon.

How to Diagnose P0204

Identify Cylinder 4

On a four-cylinder engine, cylinder 4 is the injector furthest from the timing belt, usually at the gearbox or flywheel end. Access is often the tightest of the four.

Listen for Operation

Use a stethoscope on the injector while the engine idles. The absence of regular ticking on cylinder 4 supports an open-circuit fault.

Resistance Test

Unplug and measure the injector. Infinite resistance confirms an open coil; a normal value (around 12–16 ohms petrol, 0.5–2 ohms diesel) sends you to the wiring instead.

Check Power and Earth

Back-probe the connector while cranking to confirm a 12V supply and a switching earth from the ECU. A missing signal means the fault is upstream.

Continuity Through the Loom

Test each wire from the injector plug to the ECU. Pay special attention to sections routed near the exhaust at the rear of the engine, a common spot for heat damage on cylinder 4.

Swap Test to Confirm

Exchange the cylinder 4 injector with a known-good one. A fault that moves cylinders confirms the injector; a fault that stays put confirms wiring or ECU.

Catalyst at RiskA dead cylinder 4 floods the catalytic converter with raw fuel. Sustained driving, especially at speed, can destroy the cat, so keep use to a minimum until the circuit is repaired.

Verdict

Test the cylinder 4 injector's resistance first - an open winding is the most likely cause. If it reads correctly, concentrate on the connector and the harness at the gearbox end, where heat and access make faults more likely.

Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.