P27A4 code meaning
Electric/Auxiliary Transmission Fluid Pump B Control Circuit Low
Powertrain system Generic code P27xx codes Check engine light
What P27A4 means
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.
P27A4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P27A4
- Stop-start unavailable
- MIL or transmission warning after repeated trips
- Blown fuse in the pump supply possible
Common causes of P27A4
- Most likelyControl wire insulation chafed through to ground
- PossibleWater or corrosion bridging pins to ground in the pump connector
- Less likelyInternal short to ground inside pump B
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Aux trans pump bWiring harness
What each part of P27A4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 7 | Area: transmission. |
| A4 | The exact fault in that area. |
What to do next
- Note P27A4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P27A4
What does P27A4 mean?
P27A4 means "Electric/Auxiliary Transmission Fluid Pump B Control Circuit Low". P27A4 indicates that the control line of auxiliary pump B is pulled toward ground. It is a generic code, so it means the same on every make.
Is it safe to drive with P27A4?
Drive gently and only to a workshop. With P27A4 the car can go into limp mode (reduced power) to protect itself.
Will P27A4 fail an emissions test?
Yes, it can. P27A4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P27A4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P27A4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.