What it means
The drive circuit for the air-assist pressure-control valve in the DEF system is open — atomisation pressure cannot be regulated, dosing accuracy degrades.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The air pressure control valve regulates compressed air used to atomise DEF as it is injected into the exhaust. The control module monitors the valve’s electrical drive circuit and sets P20A6 when that circuit appears open, so it cannot reliably command the valve.
Common causes
Most likely first.
- Solenoid 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
- Use a scan tool to record freeze-frame data and check DEF dosing and air-pressure command values to see when the fault occurred.
- Inspect the air pressure control valve connector for a pushed-back pin, loose fit, corrosion, or damaged wiring.
- With the circuit unpowered as directed by the manufacturer, check the valve solenoid winding for continuity and compare it with the manufacturer's specification.
- Check continuity through the valve wiring and connectors between the valve and control module, looking for an open circuit.
- If the wiring and valve test correctly, have the control module connection and any suspected internal solder joint assessed using the manufacturer's diagnostic procedure.
Common fixes
- Reconnect the plug securely or repair a pushed-back connector terminal.
- Repair an open or damaged wire in the valve harness.
- Replace the air pressure control valve if its solenoid winding is open.
- Repair or replace the affected module if an internal circuit fault is confirmed.
Can I keep driving?
The vehicle may remain drivable, but incorrect DEF atomisation can raise NOx emissions and trigger a restart countdown. Follow any dashboard 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.