What it means
The control module monitors reductant pump B when it commands the pump to run. It sets this code when the pump does not start or stops operating when expected.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant pump B circulates DEF through the SCR dosing system when commanded. The control module monitors whether pump B starts and continues operating; P20FB sets when it does not run as expected, despite no simple open-circuit condition being identified.
Common causes
Most likely first.
- Pump head crystallised — motor seized
- Carbon or debris buildup preventing free movement
- Component wear with age - mechanical play
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and review pump B command, status, and available pressure or flow data to establish whether it fails at startup or during operation.
- Inspect pump B and its lines for DEF leakage, crystallized deposits, or visible damage that could restrict or seize the pump.
- Check the pump B connector and harness for corrosion, loose pins, or damage before condemning the pump.
- Use supported scan-tool output controls to command pump B and observe its operating status and any pressure or flow response.
- If the pump does not respond, test its supply, ground, control circuit, and mechanical operation using the manufacturer's procedure and specification.
Common fixes
- Clean accessible DEF crystallization from the pump B area and connections using an approved method.
- Repair damaged wiring or poor connections to pump B.
- Replace pump B if it is seized, worn, or fails its commanded-operation test.
- Clear the fault and verify pump B operation and reductant pressure or flow after repair.
Can I keep driving?
Avoid unnecessary trips because loss of pump B can impair SCR dosing and lead to emissions warnings or a no-start countdown. If a countdown is displayed, follow the vehicle's instructions and have the fault repaired promptly.
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.