Comprehensive diagnostic guide for OBD-II P0204: Injector Circuit Malfunction - Cylinder 4

Disclaimer on data sources

• Code meaning and general OBD-II context: Wikipedia's OBD-II sections on Diagnostic Trouble Codes and Powertrain Codes provide high-level definitions of how DTCs are structured and interpreted within the OBD-II system.
• Specific code wording for injector cylinder 4: An open/open-circuit injector condition for cylinder 4 is represented in open-source definitions as (injector cylinder 4 - open circuit). Use of this definition aligns with P0204's typical interpretation as an injector-circuit fault for cylinder 4.
• Real-world symptom context: NHTSA user complaint data describes a cylinder-related misfire scenario with fuel economy impact and warning lights (EPC), illustrating how injector/electrical faults can manifest as misfire and drivability concerns. Treat this as a symptom example, not a universal pattern.
• Safety and general repair guidance: The diagnostic approach below follows general automotive diagnostic best practices and the principle that P0204 is a cylinder-4 injector-circuit issue, consistent with the sources above.

What This Code Means

• P0204 is a powertrain (P) diagnostic trouble code indicating an injector circuit malfunction in cylinder 4. The issue is electrical in nature (injector circuit) rather than a pure mechanical failure of the fuel metering itself, and it is cylinder-specific.

Symptoms

• Cylinder 4 misfire: rough running, intermittent stumble, or misfire codes may be logged with P0204.
• EPC/Check Engine lights may illuminate, along with reduced engine performance and potentially decreased fuel economy.
• In some cases, the engine may crank but fail to start or run poorly if the injector remains open/closed abnormally or if the PCM is failing to command the injector.
• Note: P0204 is cylinder-specific; if multiple cylinders misfire, a P030X pattern (misfire in a given cylinder) or P0300 (random/multiple misfires) may also appear.

Probable Causes

• Injector 4 circuit fault (open or short to ground) in the injector wiring or the injector itself - most common explanation for P0204.
• Wiring harness or electrical connector issue in the cylinder 4 injector circuit (damaged insulation, loose/ corroded pins, improper locking).
• Faulty injector (stuck, not opening due to internal mechanical issue or internal coil failure).
• PCM/ injector-driver circuitry fault (less common, but possible if several injector circuits show issues or a diagnosed driver fault is present).
  Note on data: The available NHTSA complaint data for P0204 is limited (one user complaint about misfire and related symptoms), so probability estimates are inherently uncertain. In practice, injector-circuit faults and wiring/connectors dominate the likely causes, with PCM faults being less frequent. If you have access to a broader data set, .

Diagnostic Approach

1) Verify and document

• Retrieve freeze-frame data from the vehicle's OBD-II scanner when P0204 is current or most recent.
• Check for any related codes (e.g., P0304 cylinder 4 misfire, P0300 random misfire, P0171/P0174 fuel trims, fuel pressure, etc.) to get a complete picture.
• Note any symptom timing (engine load, RPM, idle quality) and whether the EPC light is present.

2) Visual and mechanical inspection

• Inspect the injector 4 wiring harness and connector for damage, abrasion, exposed conductors, oil/grease contamination, and secure locking.
• Check for fuel system leaks around the fuel rail and near the injector; look for signs of mis-routed or pinched wiring.
• If accessible, inspect cylinder 4 for obvious mechanical issues (compression test as needed) to rule out a non-electrical cause of misfire.

3) Electrical tests on the injector circuit (cylinder 4)

• Power supply check: With ignition on, back-probe the injector supply circuit (positive side) for approximately battery voltage (12V nominal). If using a diagnostic fuse, confirm the fuse is intact and supply is present.
• Injector control signal: With an oscilloscope or a noid light, verify that the PCM is delivering a pulsed ground/signal to the injector (depending on the vehicle's injector drive architecture). Noid-light testing or scope capture should show a clean, regularly repeating duty cycle when the injector is commanded on.
• Ground path: Verify a solid, low-resistance ground path from the injector driver circuit to the PCM and engine ground. High resistance or open ground can cause intermittent or no injector operation.
• Coil resistance check: Measure the resistance of injector 4 coil (at the injector body between the two terminals). Compare with the manufacturer's spec. A resistance out of spec implies a faulty injector coil.

4) Isolate the fault

• If 12V supply is present and the injector is commanded on but no current flows, suspect an open circuit in the injector coil or a fault in the injector driver path (wiring, connector, or PCM output).
• If the injector coil resistance is out of spec, replace the injector and re-test.
• If the coil resistance is within spec but the control signal is absent or highly erratic, focus on wiring/connector integrity and PCM/injector-driver circuit integrity.
• If the coil resistance is within spec and the control signal looks correct, but the injector still does not spray or sprays inconsistently, consider replacing the injector or, if other injectors show similar symptoms, consider a PCM fault (less common but possible).

5) Optional advanced testing

• Use a physical test injector (noid light test) to visually confirm proper injector pulse width and timing.
• Use a scope to capture injector current waveform when commanded; look for a clean, repeatable waveform that corresponds with engine RPM/load.
• If available, perform a cylinder balance or misfire history test to confirm persistent cylinder 4 misfire after wiring and injector checks.

6) Mechanical checks to rule out non-electrical causes

• Perform a compression test on cylinder 4 to ensure cylinder integrity; low compression can cause misfire that might mimic injector issues.
• If compression is low, address the mechanical issue first; if compression is normal, focus on electrical/fuel delivery path.

7) Repair steps (typical, based on inspection results)

• Repair/replace wiring harness segments or connectors found damaged or corroded.
• Replace injector 4 if injector coil is open, shorted, or otherwise failed.
• Repair PCM wiring/driver path if confirmed to be defective (less common; may require software updates or PCM replacement if confirmed by professional diagnostics).
• After any repair, clear P0204, run the engine through a drive cycle, and re-scan to confirm the fault does not return.

8) Verification and road testing

• Confirm there are no misfire codes related to cylinder 4 after repairs; ensure no P0304 or related misfire codes reappear.
• Verify fuel trims return to normal and fuel economy stabilizes.
• Confirm that the EPC light does not reappear; if it does, re-evaluate the system for additional faults.

Post-repair checks and notes

• Re-check for stored codes and monitor live data to ensure injector 4 is being properly commanded and delivering fuel.
• If the code persists after injector replacement and wiring repair, re-evaluate the PCM/injector-driver circuit and consider consulting service information for your specific vehicle model (some vehicles have unique injector drive behavior or safety interlocks).

Safety Considerations

• Relieve fuel system pressure before removing injector connectors or fuel rail components when possible.
• Disconnect the battery when performing heavy electrical work or connector disassembly near the fuel rail to prevent shorts or sparks.
• Wear eye protection and have a fire extinguisher accessible in the work area; fuel is flammable.
• Follow vehicle-specific service procedures; some vehicles require special tools for injector harness removal or fuel rail servicing.

Documentation

• P0204 indicates an injector-circuit fault for cylinder 4. The fault is typically electrical (open/short in injector circuit, wiring/connector issue, or injector itself). A thorough electrical check of cylinder 4's injector circuit, including voltage, ground, coil resistance, and driver signal, is recommended after confirming the fault with scan data and live-data observation. In the limited available real-world data, injector-wiring or injector itself are more common causes than PCM faults; however, any fault within the injector drive path can cause the P0204 code and misfire symptoms as described in the NHTSA complaint example and the general OBD-II code framework.

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 1 real-world reports analyzed
• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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