P04C9 — Fuel Fill Door Open Request Sensor/Switch Performance/Stuck Off

P04C9 is stored when the ECU sees open circuit on door / window / lock. That describes how the circuit is behaving, not which part has failed — the wiring, the connector and the component itself can all produce it, which is why the checks below start with the cheapest.

Common causes

The cause is almost always one of a few things. Here they are — P04C9 rarely comes from anything outside this list:

• Broken wire, most often at a flex point such as a door hinge or engine mount
• Corroded or pushed-back terminal in the connector
• Blown fuse on the supply side
• Component failed with an internally open winding or element

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Check the relevant fuse first — it is the cheapest thing that produces this code
2. Unplug and inspect both halves of the connector for corrosion and terminal fit
3. Continuity-test the wire end to end while flexing the loom by hand
4. Measure the component itself against its specification
5. Repair the break or replace the component, whichever the measurement points to

That is what holds for P04C9 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.