What it means
This code indicates a high voltage condition in the reductant injection air pressure control valve circuit.
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 in the exhaust. P20A9 sets when the control module detects voltage higher than expected in the valve circuit, which can result from a short to battery positive or a supply wire contacting a positive source.
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 live valve command and circuit readings to identify when the high signal is present.
- Inspect the valve harness for pinched wiring or damaged insulation near power feeds that could connect the circuit to battery positive.
- Check connector terminals for corrosion, looseness, or damage that could distort the circuit reading.
- With power isolated according to the manufacturer's procedure, test for a short between the valve circuit and battery-positive wiring.
- Verify wiring continuity and valve operation against the manufacturer's specification before testing the control module.
Common fixes
- Repair a loose, corroded, or damaged valve connector.
- Free and protect wiring that has been pinched against a positive supply.
- Repair the harness where the valve circuit is shorted to battery positive.
- Replace the valve or repair the control module only if tests confirm an internal fault.
Can I keep driving?
The vehicle may remain driveable, but incorrect air control can impair DEF dosing, increase NOx emissions, and lead to a restart countdown. Get the circuit checked promptly and follow any dashboard countdown.
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.