Powertrain, generic (extended)

P27A4 Electric/Auxiliary Transmission Fluid Pump B Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P27A4 indicates that the control line of auxiliary pump B is pulled toward ground. The module checks the line while its driver is switched on; if the measured level stays close to zero, the circuit counts as shorted low. No manufacturer documents P27A4 publicly. GM's 2025 TCM tables give the mirror test for pump A as P2798: under 0.5 Ω between signal and controller ground in more than 20 of 25 samples at 100 ms spacing, evaluated only with battery voltage between 9 and 15 V and the feeding high-side driver on; it is Type B, so the MIL follows after two trips. A short of this kind can also blow a fuse or trip the driver's overcurrent protection. Check for insulation worn through against the housing or subframe and for water in connectors. P27A5 covers the high side.

Likely causes

Control wire insulation chafed through to ground
Water or corrosion bridging pins to ground in the pump connector
Internal short to ground inside pump B

Symptoms

  • Stop-start unavailable
  • MIL or transmission warning after repeated trips
  • Blown fuse in the pump supply possible

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–900
Time0.5–3 h

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

Parts involved

aux trans pump bwiring harness

Related codes

P27A2P27A5P2798P276B

References: gsi.ext.gm.com, static.nhtsa.gov, law.cornell.edu.