U300F — Ignition Input Accessory

Diagnostic algorithm

1. Retrieve and document DTC(s) and related data with a capable scan tool; note ignition position and battery voltage state.
2. Visually inspect ignition switch, connectors, wiring routes, and ground points for damage, corrosion, or modifications.
3. Verify fuses and ACC relay are present and functional; replace or bench-test if suspect.
4. With a multimeter, check ACC terminal voltage at the ignition switch in OFF, ACC, and ON positions. Expected: ~0 V (OFF) and battery voltage when ACC/ON.
5. Back-probe the BCM/gateway ACC input pin while operating the key and compare voltage changes to the ignition switch reading to locate open/short.
6. Check continuity and resistance between the ignition switch ACC terminal and the BCM input; repair any open or high-resistance circuits.
7. Use an oscilloscope or CAN diagnostic tool to confirm the BCM/gateway is sending/receiving the ignition/accessory state on the vehicle network; inspect for bus errors.
8. If wiring and fuses are good but input never changes at the BCM, consider BCM input circuit fault — confirm with wiring diagram and perform connector pin voltage/ground tests.
9. If module replacement is required, verify programming/calibration status and reprogram with OEM software as needed. After repairs, clear codes and re-test vehicle operation across multiple key cycles.
10. Document repairs and re-check for any intermittent faults; advise customer of any aftermarket accessories found and remedial action taken.