What it means
The auxiliary transmission fluid pump B falls below the calibrated minimum speed for the commanded duty. Mechanical resistance high or supply voltage 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 supplies fluid flow when the transmission control system calls for it. The TCM sets P28EE when pump B runs below the minimum speed for the commanded duty, which can result from a binding pump or low supply voltage under load.
Common causes
Most likely first.
- Pump mechanically binding
- Supply voltage low under 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
- Review freeze-frame and live data to compare pump B's commanded duty with its measured speed and note whether the underspeed occurs under load.
- Inspect pump B's fuse, power feed, ground, connector, and harness for looseness, corrosion, heat damage, or voltage drop causes.
- Measure supply voltage at pump B while it is commanded on and compare the result with the manufacturer's specification to identify voltage loss under load.
- Check the transmission fluid condition and level according to the manufacturer's procedure, then listen or feel for signs of pump binding during a supported command test.
- If electrical supply and fluid checks pass, inspect pump B for mechanical resistance or binding using the manufacturer's service procedure.
Common fixes
- Repair a poor power or ground connection causing low voltage at pump B under load.
- Repair damaged wiring, terminals, or a failed fuse in the pump B supply circuit.
- Correct fluid level or condition if it is contributing to pump resistance, following the manufacturer's procedure.
- Replace pump B if testing confirms that it is mechanically binding.
Can I keep driving?
Avoid extended driving or heavy loads if shifts are delayed, harsh, or slipping, because underspeed can reduce the fluid flow the transmission expects. If symptoms worsen or the transmission enters limp mode, stop driving and arrange prompt diagnosis to limit 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.