Powertrain, generic (extended)

P2851 Shift Fork Position Sensor A/B Correlation

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Shared supply or ground of the A and B sensors disturbed
One of the two sensors drifting while still in range
Corroded contacts at the internal transmission connector

Symptoms

  • Gears of forks A and B locked out
  • Warning lamp, limp-home mode

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,500
Time1–8 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

shift fork position sensor ashift fork position sensor btcm

Related codes

P2852P2832P2837P2845P2846

References: law.cornell.edu, gsi.ext.gm.com, fordservicecontent.com.