What it means
The DEF system air-assist pressure exceeds the upper threshold — pressure-control valve stuck closed or air pump runaway.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
An air-assisted DEF system uses an air pump and pressure-control valve to provide the air pressure needed for reductant delivery. This code sets when monitored air pressure exceeds the upper limit, which can happen if the control valve stays closed or the pump does not regulate as commanded.
Common causes
Most likely first.
- Pressure control valve stuck closed
- Air pump does not regulate
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
- Capture freeze-frame data and compare commanded with measured reductant air pressure on a scan tool to confirm the over-pressure condition and when it occurs.
- Inspect the air lines for blockage, pinching, or damage and check connectors for the pump, valve, and pressure sensor.
- Check the pressure-sensor reading with the system inactive and during operation for values that are implausible or do not respond as pressure changes.
- Use an available scan-tool actuator test to command the pump and pressure-control valve, observing whether pressure responds as expected.
- Test the valve, pump control circuit, and pressure-sensor circuit to the manufacturer’s specification.
- If the components and circuits pass, check vehicle-specific service information for a control-module fault or update.
Common fixes
- Clear a confirmed restriction or repair a damaged air line or fitting.
- Repair a loose or damaged connector or wiring at the pump, valve, or pressure sensor.
- Replace a pressure-control valve that fails its specified operation test.
- Repair or replace an air pump that fails to regulate pressure.
- Replace a pressure sensor or address a control-module fault only after circuit testing confirms the cause.
Can I keep driving?
A short, cautious drive may be possible if the vehicle runs normally, but have the system checked soon because reductant delivery and NOx control may be affected. Do not ignore a DEF or starting-restriction countdown, as the vehicle may eventually limit or prevent restarting.
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.