What it means
The auxiliary transmission fluid pump B exceeds the calibrated maximum speed for the commanded duty. Mechanical load on the pump unexpectedly low.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Auxiliary transmission fluid pump B circulates fluid to support transmission operation when required. The TCM monitors pump speed against its commanded duty and sets P28ED when pump B spins faster than its calibrated maximum, which can indicate lost mechanical load from a damaged impeller or aerated fluid.
Common causes
Most likely first.
- Pump impeller damage - load lost
- Aerated fluid - reduced load
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Check freeze-frame and live data for pump B commanded duty, measured speed, and whether the overspeed occurs continuously or only under particular conditions.
- Inspect the transmission fluid condition and level using the manufacturer's procedure, looking for signs of aeration, contamination, or an incorrect fill condition.
- Inspect pump B wiring and connectors for damage or poor connections that could make the speed feedback inaccurate.
- Use a scan tool to command pump B if supported and compare its speed response with the manufacturer's specification while checking for abnormal noise or vibration.
- If fluid and electrical checks pass, inspect pump B and its impeller for damage or loss of load using the manufacturer's service procedure.
Common fixes
- Correct the transmission fluid level or condition if the manufacturer's inspection identifies a fluid problem causing aeration.
- Repair damaged pump B speed-sensor wiring or connector faults affecting the reported speed.
- Replace pump B if inspection confirms impeller damage or loss of mechanical load.
- Refill or service the transmission fluid as specified after pump repair, then verify pump speed under command.
Can I keep driving?
Limit driving and avoid heavy loads if shifts are harsh or the transmission is slipping, since abnormal pump speed can mean fluid circulation is not as expected. Stop driving if transmission operation worsens and arrange prompt diagnosis to reduce the risk of internal damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.