What it means
DEF pump B control line stuck high — short to B+. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The pump B control circuit carries the command that operates the second DEF pump on equipped systems. The control module sets P20FD when it detects the circuit held above its expected electrical level, commonly because the control wire is shorted to battery positive.
Common causes
Most likely first.
- Short to B+
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
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 check pump B command and circuit status in live data to confirm when the high condition appears.
- Inspect the pump B harness for pinched or chafed sections that could contact a battery-positive wire or supply.
- Check the pump B connector and module connector for corrosion, moisture, displaced pins, or signs of unwanted power entering the control circuit.
- With the circuit isolated as directed by the manufacturer, test the pump B control wire for a short to power and check terminal fit.
- If the harness is sound, test the pump B driver output at the control module using the manufacturer's procedure and specification.
Common fixes
- Repair pump B control wiring shorted to a battery-positive source.
- Replace damaged insulation or reroute wiring away from the power source causing the short.
- Repair corroded or damaged connector terminals and restore connector seals.
- Repair or replace the control module only if its driver is confirmed faulty.
Can I keep driving?
The engine may still run, but pump B control and SCR dosing can be disrupted, affecting emissions and potentially triggering a no-start countdown. Avoid extended driving and follow any countdown warning.
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.