What it means
The DEF injection valve at bank 1 unit 1 does not close when commanded — DEF flows continuously into the SCR. Risk of crystallisation in the catalyst and over-dosing.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
The bank 1, unit 1 reductant injector meters DEF into the SCR exhaust stream to help reduce NOx. The control module checks whether this injector stops dosing when commanded closed; it sets P20F6 when it detects continued flow from that specific valve.
Common causes
Most likely first.
- Valve seat eroded or contaminated
- Valve plunger seized open
- Carbon or debris buildup preventing free movement
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 SCR live data, including commanded dosing and any available reductant pressure or flow readings, to see when unwanted dosing is detected.
- Inspect the bank 1, unit 1 injector and nearby exhaust for wet DEF, white crystals, or deposits that may indicate leakage or a valve that is not seating.
- Check the injector connector and wiring for corrosion, loose pins, or damage that could interfere with its operation.
- Use the scan tool's supported output controls to compare commanded dosing with the response from the bank 1, unit 1 injector.
- If the valve continues to pass DEF when commanded closed, test the injector and its control circuit using the manufacturer's procedure and specification.
Common fixes
- Clean DEF deposits from the injector area and connector, if contamination is present and cleaning is permitted by the manufacturer.
- Repair damaged wiring or connector pins at the bank 1, unit 1 injector.
- Replace the bank 1, unit 1 reductant injector if its seat or plunger fails testing.
- Remove DEF crystals from affected SCR components using the manufacturer's approved procedure and assess for damage.
Can I keep driving?
Avoid extended driving because continuous DEF dosing can cause over-dosing and deposits in the SCR catalyst. Follow any vehicle warning about a no-start countdown and arrange repair 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.