What does it mean?
B1850 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Driver Seat Lumbar Horizontal Position Sensor Circuit .
Symptoms
- Lumbar support does not stop where it should and runs to the end of travel
- Memory recall no longer restores the lumbar setting
- Lumbar moves erratically or in short steps
- Position value on the scan tool is frozen or jumps around
- The rest of the seat memory functions work normally
Likely causes
- Position sensor inside the lumbar unit worn or out of its calibrated range
- Sensor connector under the seat loose, dirty or corroded
- Open or short circuit in the sensor signal or reference wire
- Under-seat harness pinched by the rails at full seat travel
- Lumbar mechanism jammed so the sensor never reaches the expected position
- Sensor reference voltage lost because of a shared five volt circuit fault
- Seat control module internal fault
How to fix it
- Read the lumbar position value on a scan tool while adjusting it slowly
- Unplug the sensor and check whether the reference voltage returns to normal
- Test the signal and reference wires for opens and shorts with both ends disconnected
- Confirm that other sensors sharing the same reference are still reading correctly
- Move the seat to both extremes and inspect the harness for pinching
- Free or replace the lumbar mechanism if it binds mechanically
- Replace the sensor or the lumbar assembly and run the seat calibration afterwards