P2851Shift Fork Position Sensor A/B Correlation
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P2851 is meant for a plausibility comparison between the position sensors of forks A and B. Both readings may be in range on their own, yet together they describe an impossible state, such as both forks engaged at once, or they move in step in a way that points to a shared supply or ground fault. No maker publishes a strategy for this pair code. In GM's Corvette dual-clutch software, rods 1 and 2 on the odd shaft do share a sensor reference supply, but faults are kept per rod as P2832 and P2837; Ford's TR9070 likewise assigns one consistency code to each rod sensor. CARB 13 CCR 1968.2 requires the rationality check without prescribing the pair form. Expect the module to drop the odd or affected gears until the cause is found.
- What you may notice
- Gears of forks A and B locked out
- Warning lamp, limp-home mode
- Likely causes
- LikelyShared supply or ground of the A and B sensors disturbed
- PossibleOne of the two sensors drifting while still in range
- PossibleCorroded contacts at the internal transmission connector
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.