What it means
DEF pump B control line stuck low — short to ground. 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 P20FC when it detects the circuit held below its expected electrical level, commonly due to a short to ground.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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 see whether the low condition is continuous or intermittent.
- Inspect the pump B connector and harness for chafing, trapped wiring, moisture, corrosion, or pin damage that could connect the control line to ground.
- Wiggle the accessible pump B harness while monitoring the fault status to help locate an intermittent short, if this can be done safely.
- With the circuit isolated as directed by the manufacturer, check the pump B control wire for a short to ground and inspect connector pin 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 chafed pump B control wiring that is shorted to ground.
- Clean or replace corroded pump B connector terminals and restore a sound seal.
- Correct pin damage or poor terminal fit at the pump or module connector.
- Repair or replace the control module only if its driver is confirmed to hold the circuit low.
Can I keep driving?
The engine may still run, but pump B and SCR dosing may be unavailable, 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.