Pump B is a second electrically driven transmission fluid pump that works independently of the engine, for example to hold pressure during start-stop or electric driving, or to feed clutch cooling. P28ED says its motor runs faster than commanded or faster than its permitted maximum. That typically happens when the pump loses its load because it draws air, at low fluid level or with a blocked suction filter, or when the speed estimate of a sensorless motor goes wrong. No manufacturer documents P28ED publicly. Analogous checks exist for single pumps: GM logs P179A on the 2016 Cadillac ELR when the pump motor exceeds its maximum speed of 6500 rpm, Type A, and Ford's 2019 plug-in hybrids flag a speed above 105 percent of command for 3 s under P0B0D. CARB requires functional checks.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
Symptoms
- Transmission warning or check engine light on
- Start-stop or electric driving disabled
- Whining from the pump area
Most likely causes
- Low fluid level or air drawn in, so the pump runs without load
- Blocked suction filter or inlet
- Faulty speed feedback or control in the pump controller
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P28ED means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 8: Transmission
- ED: Specific fault in that system
Frequently asked questions
Can I keep driving with code P28ED?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P28ED?
The most common causes are: low fluid level or air drawn in, so the pump runs without load; blocked suction filter or inlet; faulty speed feedback or control in the pump controller.
Related codes
- P28EEElectric/Auxiliary Transmission Fluid Pump B Underspeed
- P277CElectric/Auxiliary Transmission Fluid Pump B Motor Stalled
- P27A3Electric/Auxiliary Transmission Fluid Pump B Performance/Stuck Off
- P276DElectric/Auxiliary Transmission Fluid Pump B Motor Current
Sources
- gsi.ext.gm.com: 16OBDG01 Hybrid Diagnostics
- fordservicecontent.com: OBDSM1901 HEV
- law.cornell.edu: 13 CCR 1968.2
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.