B2101 — 6 documented meanings by make

B2101 does not mean the same thing on every car — manufacturers reuse the number for different faults. 4 makes document it: Chevrolet (USA), Chrysler, Mazda, Saab. Find your make below.

B2101 on Chevrolet (USA)

On Chevrolet (USA) vehicles B2101 is documented as: Recline backward switch failed.

Related Chevrolet (USA) codes:

B2101 on Chrysler

On Chrysler vehicles B2101 is documented as: Ignition run/start input circuit low. B2101 is stored when the ECU sees signal too low 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: High resistance in the supply or ground path, dragging the signal down; Corroded connector adding resistance the module reads as a low reading; Sensor drifted out of tolerance with age; Partial short to ground somewhere in the signal line.

Check first, cheapest first:

1. Read the live value on a scanner and compare with a known-good reading
2. Clean and re-seat the connector, then re-test before replacing anything
3. Check the ground point for that circuit — a poor ground causes this far more often than a bad sensor
4. Measure the sensor against specification with the connector back-probed
5. Replace the sensor only once the wiring and ground have been ruled out

Related Chrysler codes:

B2101 on Mazda

On Mazda vehicles B2101 is documented as: Head restraint switch circuit. B2101 points at airbag / restraint system. The standard definition names the system involved; which part inside it has actually failed is what the checks below establish, cheapest first.

Related Mazda codes:

B2101 on Saab

On Saab vehicles B2101 is documented as: Legroom adjustment Hall sensor, open circuit; Legroom adjustment Hall sensor, short circuit to ground; Legroom adjustment Hall sensor, short circuit to positive. B2101 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 Saab 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 B2101 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.