P0204

Injector Circuit Malfunction - Cylinder 4

P0204 is a generic OBD-II powertrain diagnostic trouble code: Injector Circuit Malfunction - Cylinder 4. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0204
Group
Powertrain
System
Fuel/Inj
Severity
high
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What P0204 means

P0204 is stored when the Engine Control Module detects an electrical fault in the fuel injector circuit for cylinder 4. The ECM commands each injector in sequence according to the engine's firing order and monitors the resulting voltage and current waveform on the control wire. A reading that deviates from the expected solenoid response – indicating an open circuit, short circuit, or abnormal resistance – causes the module to flag P0204 and illuminate the check engine light.

On four-cylinder engines, cylinder 4 is usually the rearmost cylinder (furthest from the front of the engine), which means its wiring harness is often routed across or near other components. On V6 and V8 engines the position depends on manufacturer numbering conventions. Physical harness routing near hot or moving parts can make cylinder 4 wiring particularly vulnerable to heat damage or abrasion over time, making a thorough visual harness inspection especially important.

With cylinder 4's injector circuit open or shorted, fuel delivery to that cylinder is compromised. The engine misfires noticeably, typically setting a companion P0304 code, and the driver experiences rough idle, hesitation, and power reduction. On turbocharged engines the uneven combustion can cause additional stress on the turbocharger and exhaust components. The catalytic converter is also at risk if the misfire persists, as unburned fuel entering the converter causes thermal damage.

Standard diagnostic protocol applies: harness and connector inspection, injector coil resistance measurement, noid-light pulse test, circuit continuity check back to the ECM, and an injector swap between cylinders to determine whether the fault is in the injector or the wiring. ECM failure is a last-resort diagnosis after all external circuit components are verified good.

Common causes

Most-frequently reported root causes when P0204 is logged.

  • 1
    Faulty cylinder 4 fuel injector (failed solenoid coil, internal open or short)
  • 2
    Damaged, pinched, or heat-cracked wiring in the cylinder 4 injector harness
  • 3
    Corroded or loose connector at the cylinder 4 injector
  • 4
    Short to ground in the cylinder 4 injector signal wire
  • 5
    Open circuit in the power supply or ground return to injector 4
  • 6
    Blown injector circuit fuse or failed relay
  • 7
    Terminal corrosion inside the injector connector from moisture
  • 8
    ECM/PCM internal driver failure for cylinder 4 channel (uncommon)

Symptoms drivers notice

Check engine light illuminated
Cylinder 4 misfire, typically with companion code P0304
Rough idle and noticeable vibration
Reduced engine power and sluggish acceleration
Increased fuel consumption
Fuel odour from exhaust
Hard starting or intermittent stalling
Possible increase in short-term fuel trim values on scan tool

How to diagnose P0204

A typical diagnostic flow when this code is present.

  1. 1
    Scan for all DTCs and save freeze-frame data; identify any companion misfire or fuel-trim codes alongside P0204.
  2. 2
    Visually inspect the cylinder 4 injector wiring harness end-to-end, paying attention to areas near exhaust manifolds, heat shields, and grommets where chafing and cracking are common.
  3. 3
    Inspect and replace any blown fuses or failed relays associated with the fuel injection system.
  4. 4
    Measure cylinder 4 injector coil resistance and compare to the OEM specification; if resistance is open (infinite) or outside tolerance, the injector is faulty.
  5. 5
    Insert a noid light into the disconnected injector connector and crank the engine – flashing confirms an ECM activation pulse; no flash narrows the fault to the wiring or ECM.
  6. 6
    Test signal wire continuity and check for shorts to ground or battery voltage between the injector connector and the ECM.
  7. 7
    Swap the cylinder 4 injector with a known-good injector from another position; clear codes and test drive. If P0204 re-appears on the original cylinder with the different injector, the wiring or ECM is at fault. If the code follows the injector to its new position, replace the injector.
  8. 8
    If the circuit and injector pass all tests, inspect the ECM connector for pushed-back pins and evaluate ECM driver output for cylinder 4 before authorising ECM replacement.

Related powertrain codes

Frequently asked questions

P0204 appeared right after a spark plug service – is that related?

Possibly. During spark plug service the injector harness near cylinder 4 can be disturbed, stretching or cracking a marginally healthy wire or unseating a connector. Inspect all connectors and harness sections that were moved during the service.

My scan tool shows P0204 and P0304 together – which do I fix first?

P0204 is the root cause; P0304 is the consequence. Resolve the injector circuit fault (P0204) and the cylinder 4 misfire (P0304) will typically clear on its own once fuel delivery is restored.

Can ethanol or fuel additives damage the injector and cause P0204?

High-ethanol blends beyond the vehicle's specification, or aggressive fuel system cleaners used repeatedly, can degrade injector seals and coil windings over time, potentially causing electrical failure. Always use fuel grades within the manufacturer's recommendation.

How accurate is the injector swap test?

It is highly reliable for distinguishing injector hardware failure from a wiring fault. The only caveat is that on some engines injectors are not interchangeable between positions due to different flow-rate calibrations or connector types, so confirm compatibility before swapping.

Does P0204 affect all driving conditions equally or only under load?

The fault is present at all times when the circuit is open, but the misfire may be more noticeable under load (acceleration, hills) because the demand for fuel from all cylinders is higher. At idle with a light load the remaining cylinders can partially compensate.

Disabling P0204 in software

RaceTune can permanently disable P0204 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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