P0204 — Injector Circuit - Cylinder 4

Diagnostic algorithm

1. Retrieve and record codes and freeze-frame data. Check for related misfire or fuel codes.
2. Visually inspect the injector #4 connector and harness for corrosion, bent pins, melted insulation or oil/water contamination.
3. With ignition OFF disconnect the injector connector and measure injector coil resistance at the injector leads; compare to spec.
4. With key ON, verify there is battery voltage at the injector feed circuit (usually present at all injector connectors). Check related fuse/relay.
5. Backprobe the injector control pin while cranking and/or idling. Using a DMM or oscilloscope verify the ECM is commanding the injector (pulsed signal).
6. If no command signal from ECM, check continuity from the injector connector control pin back to the ECM pin; look for opens or shorts to battery/ground.
7. If command present but injector not operating or resistance out of range, swap injector #4 with a known-good injector from another cylinder and observe if code follows the injector.
8. If swapped injector cures the code, replace the failed injector. If the code follows the wiring, repair or replace the affected harness/connector.
9. If wiring and injector check good but no ECM driver output (or short at ECM pin), perform ECM bench tests or refer to manufacturer procedures—ECM replacement is last resort.
10. After repair, clear codes and perform test drive to confirm code does not return and symptoms are resolved.