P27A4Electric/Auxiliary Transmission Fluid Pump B Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Stop-start unavailable
- MIL or transmission warning after repeated trips
- Blown fuse in the pump supply possible
- Likely causes
- LikelyControl wire insulation chafed through to ground
- PossibleWater or corrosion bridging pins to ground in the pump connector
- RareInternal short to ground inside pump B
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.