B1804 — 2 documented meanings by make

B1804 does not mean the same thing on every car — manufacturers reuse the number for different faults. 3 makes document it: Chrysler, Ford, Mazda. Find your make below.

B1804 on Chrysler

On Chrysler vehicles B1804 is documented as: Driver's door lock/unlock switch circuit stuck in unlock. B1804 is stored when the ECU sees stuck or jammed 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.

Usually one of: Carbon or oil deposits binding the mechanism; Actuator motor or solenoid failed; Linkage worn or seized; Control side working correctly but the part cannot physically move.

Check first, cheapest first:

1. Command the actuator with a scanner and watch whether it moves at all
2. Remove and inspect for deposits — cleaning frequently fixes this class outright
3. Check the electrical side is delivering the command before condemning the mechanism
4. Inspect the linkage for wear or seizure
5. Replace the actuator if it will not move under command with good supply

Related Chrysler codes:

B1804 on Ford, Mazda

On Ford, Mazda vehicles B1804 is documented as: Rear turn signal light output, short circuit to positive. B1804 is stored when the ECU sees short to positive on the circuit named in the code (rear). 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.

Usually one of: 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.

Check first, cheapest first:

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

Related Ford, Mazda codes:

Which one is yours

A generic OBD reader gives you the number, not which of these meanings your manufacturer assigned to it. To get to the specific one:

1. Use a brand-specific scanner — VCDS for VAG, INPA or ISTA for BMW, XENTRY for Mercedes, Forscan for Ford. It shows the manufacturer's own text rather than the generic one.
2. Read all stored codes together — the neighbouring ones usually point at which system is involved and narrow the list.
3. Check freeze-frame data: the RPM, temperature and load at the moment it triggered indicate which system was active.
4. If you have no brand scanner, tell me the make and model below and I will tell you which of these applies to your car.

Code family

B1xxx is body systems, manufacturer-specific.

Severity

Manufacturer-specific body system — needs a brand scanner to read the actual cause.

That is what holds for B1804 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.