P27F1: Transmission Range Control B Position Sensor/Switch Circuit Low
What to do
Apply the parking brake every time you park, and allow for the display being wrong. The module can no longer fully confirm which range the gearbox is in, so treat the selector display with suspicion until it is repaired — check the car is truly holding before you step away. If it refuses to leave park or to crank, that is the same caution, not a second fault.
What this code means
Transmission range control position sensor B is reading low.
A circuit reading low usually means a short to ground — the control line is being pulled down regardless of what the module commands, and the module stops trusting it.
The position sensor is how the module knows the shift it commanded actually happened. Shift-by-wire has no cable and therefore no mechanical certainty: the selector asks, an actuator moves the range mechanism, and this sensor reports where the mechanism truly ended up. Command and confirmation are separate circuits on purpose, and this family is the confirmation side.
Losing confirmation is why the display misbehaves. A selector showing the wrong gear, a flashing P, refusal to leave park, or an engine that will not crank in park with everything mechanically fine — all of it is the module declining to guess where the gearbox is. It would rather refuse than act on a range it cannot verify, and with park involved, that caution is correct.
Two sensors share the job, and the letter names which one. Designs carry A and B for redundancy: with one lost, the module may carry on watchfully; with the two disagreeing, it protects. The sensors themselves fail the way all position sensors do — wear on the track, water in the connector, a harness chafed where it flexes.
Likely causes
Start here
- Faulty transmission range sensor · specialist
- Corroded or loose connector · specialist
- Damaged wiring harness · specialist
Rare
- Faulty transmission control module · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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