Powertrain, generic (extended)

P27F1 Transmission Range Control B Position Sensor/Switch Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P27F1 reports a valve B position signal that has collapsed toward ground in GM's internal range selection. GM describes the trigger as a ground short or open circuit on the mode valve B sensor line and uses the same 0.414 V limit, 0.100 s debounce and 9 V minimum supply as on valve A; both table editions, 2018 and 2020, rate it Type A, MIL on the first trip. As long as P27F1 is active, the TCM cannot verify valve B and drops its movement test P27F0. Hall sensors need a supply and a ground, so a dead supply pin also produces a low reading. Typical repairs concern the internal harness, the pass-through seal or the sensor. Valve A's counterpart is P27ED; high is P27F2.

Likely causes

Valve B sensor line shorted to ground
Sensor supply missing at the internal connector
Hall sensor on mode valve B failed

Symptoms

  • MIL comes on on the first trip
  • Neutral or drive engagement slow or blocked

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–2,000
Time1–6 h

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

Parts involved

range control position sensor bwiring harnesstcm

Related codes

P27EFP27F0P27F2P27ED

References: gsi.ext.gm.com, gsi.ext.gm.com.