What it means
This code indicates a low 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. P20A8 sets when the control module sees voltage lower than expected in the valve circuit, which can happen if the circuit is shorted to ground or has excessive resistance.
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
- Save freeze-frame data and review live valve command and circuit data to see whether the low reading occurs continuously or under a particular condition.
- Inspect the valve connector and harness for corrosion, damaged insulation, or chafing that could let the circuit contact ground.
- Check the connector terminals for corrosion or poor contact, especially where resistance could reduce the circuit reading.
- With power isolated as directed by the manufacturer, test the valve circuit for an unintended short to ground.
- Check wiring continuity and valve operation against the manufacturer's specification before considering a control module fault.
Common fixes
- Clean or repair corroded connector terminals and restore a secure connection.
- Repair harness insulation where it has chafed or contacted ground.
- Replace a damaged section of valve wiring that is shorted to ground.
- Replace the valve or repair the control module only if testing confirms the component is faulty.
Can I keep driving?
The vehicle may still run, but the valve may not regulate DEF atomising air correctly, increasing NOx emissions and possibly starting a restart countdown. Avoid ignoring any countdown or emissions 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.