C1966 — RDO, short circuit to battery

C1966 is stored when the ECU sees short to positive on charging system. 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 — C1966 rarely comes from anything outside this list:

• Signal wire touching a permanently live wire in a damaged loom
• Connector pins bridged by corrosion or a bent terminal
• Internal fault in the component shorting the signal to supply
• Incorrect wiring after an aftermarket accessory was fitted

Where to start

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

1. Disconnect the component and check whether the signal line still reads battery voltage
2. Inspect the loom for melted or rubbed insulation, especially near heat sources
3. Check the connector for a pushed-back or bent terminal
4. Trace the signal wire back to the module, looking for a live wire running alongside
5. Repair the harness, or replace the component if it is shorting internally

That is what holds for C1966 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.