Powertrain, generic · Hybrid propulsion

P0C28 Electric/Auxiliary Transmission Fluid Pump A Motor Current High

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P0C28 is the overcurrent protection of auxiliary pump motor A. GM uses it on Volt, ELR and later hybrids: the inverter trips when any of the U, V or W phases exceeds 35 A, in 2016 within about 4 ms of a 62 ms window, or when the d-axis current falls below a stator temperature limit, Type A, MIL on the first trip. GM service update 18199 for the 2018 Cadillac CT6 plug-in traces P0C28 to a faulty pump restart logic in the software. Ford's TSB 18-2282 lists P0C27 and/or P0C28 on the 2013-17 Fusion Energi and C-Max Energi, fixed by reprogramming the PCM and hybrid module; Ford also reports P0C28 in the SOBDMC of the 2024-25 Mustang Mach-E LFP, solved by software. P0C27 is the low side.

Likely causes

Pump control software faulty (Cadillac CT6 update 18199, Ford Energi TSB 18-2282)

Pump seized, contaminated or running against blocked flow

Short between motor phases or to ground

Current sensing or power stage in the pump inverter faulty

Symptoms

MIL or service wrench lamp
Auxiliary pump shuts down, no auto-stop or electric mode

Repair estimate

DifficultyHardWorkshop job

Parts and labour€50–2,500

Time0.5–6 h

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

Parts involved

aux trans pump a motor

References: gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.