What it means
The control line driving DEF pump B is open. On large engines or dual-tank systems a second pump (B) supplements pump A; with B offline the system loses redundancy.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant pump B supplies DEF in systems equipped with a second pump, such as some large-engine or dual-tank configurations. The control module monitors pump B's control circuit; P20FA sets when that circuit is open and the pump cannot be driven as expected.
Common causes
Most likely first.
- Pump motor winding open
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
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 SCR live data for pump B command and any available status or pressure feedback to confirm when the circuit fault occurs.
- Inspect pump B's connector for a pushed-back pin, corrosion, moisture, or a loose fit.
- Trace the pump B harness for breaks, chafing, or heat damage, paying particular attention near the pump and module connectors.
- With the system safely disabled as specified by the manufacturer, check continuity through the pump B motor winding and control wiring.
- If the wiring and pump winding pass, check the module connector and driver output using the manufacturer's diagnostic procedure.
Common fixes
- Reseat or replace a backed-out or damaged pump B connector terminal.
- Repair an open or damaged section of pump B wiring.
- Replace pump B if its motor winding is open.
- Repair the control module only if its output is confirmed faulty after circuit checks.
Can I keep driving?
The vehicle may continue running, but SCR operation and emissions compliance can be affected, and a DEF warning or no-start countdown may follow. Limit unnecessary driving and heed any countdown or warning messages.
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.