Powertrain, generic (extended)

P27A1 Electric/Auxiliary Transmission Fluid Pump A Stuck On

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P27A1 means auxiliary transmission fluid pump A keeps running although the controller has not asked for it, or the module still sees current or rotation with the command off. Such electric pumps hold pressure in the gearbox while the engine rests during stop-start or hybrid operation. A pump that cannot be switched off usually points to a high-side driver or relay stuck closed, a control line shorted to battery voltage, or pump electronics that ignore the command. No manufacturer documents P27A1 publicly. Ford's 2024 summary for the 10R60/10R80 and 8F57 gearboxes shows that the smart pump returns its own speed and current, which is what makes an unwanted run visible; GM's 2025 tables watch the pump drivers for shorts (P2798, P2799). Expect extra battery drain and pump noise.

Likely causes

High-side driver or pump relay stays closed
Pump control or enable line shorted to battery voltage
Integrated pump electronics ignore the off command

Symptoms

  • Pump hum from the gearbox after the engine or ignition is off
  • 12 V battery discharged after standing
  • Stop-start disabled and warning shown

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–1,200
Time0.5–4 h

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

Parts involved

aux trans pump acontrol modulewiring harness

Related codes

P2796P2797P2798P2799P27A6

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