B2120 — 3 documented meanings by make

B2120 does not mean the same thing on every car — manufacturers reuse the number for different faults. 3 makes document it: Chevrolet (USA), Chrysler, Mazda. Find your make below.

B2120 on Chevrolet (USA)

On Chevrolet (USA) vehicles B2120 is documented as: Lumbar up/down sensor failed.

Related Chevrolet (USA) codes:

B2120 on Chrysler

On Chrysler vehicles B2120 is documented as: Ignition run/ACC/PAD control circuit open. B2120 is stored when the ECU sees open circuit on the circuit named in the code. 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: 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.

Check first, cheapest first:

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

Related Chrysler codes:

B2120 on Mazda

On Mazda vehicles B2120 is documented as: Passenger's door set switch stuck failure. B2120 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 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

B2xxx is body systems, manufacturer-specific.

Severity

Manufacturer-specific body system — usually doors, windows, lighting or comfort functions.

That is what holds for B2120 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.