P2861: Shift Fork B Actuator Circuit Low
What to do
Expect missing or refused gears rather than total failure — the box is declining to use gears it cannot confirm. Driving on briefly is usually safe in the gears it still offers, but have it diagnosed soon: what starts as a sensor reading can also be the first sign of a fork or synchroniser wearing, and those get more expensive with every hard shift.
What this code means
Shift fork B's actuator circuit is reading low.
A constant low usually means a short to ground — the line is pulled down regardless of what the sensor or module does, and the module stops trusting anything the circuit says.
Each shift rail has muscle behind it, and this circuit is that muscle's wiring. The fork actuator — electric motor or hydraulic piston, by design — is what actually slides the fork when the module commands a gear. Its circuit failing strands the rail where it stands: the gears it serves stop being selectable, and the module shifts around the hole.
Prove the wiring before the actuator, and the actuator before the gearbox. These circuits live in engine-bay heat and vibration; connectors corrode and harnesses chafe long before windings die. A stuck rail can also be mechanical — the stuck-fork codes exist separately for exactly that reason, and which of the two is stored decides whether this is an electrical repair or an internal one.
Likely causes
Start here
- Wiring shorted to ground · specialist
- Faulty gear shift actuator · specialist
- Open circuit in actuator wiring · specialist
- Corroded transmission connector · specialist
Less common
- Damaged transmission wiring harness · specialist
Rare
- Faulty transmission control module · specialist
Symptoms
Components involved
Information, not instruction
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