What it means
A general electrical fault is reported on the DEF system pressure sensor — used to verify pump priming, line pressure, and to detect leaks via pressure-decay tests.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant pressure sensor monitors pressure in the DEF system so the control module can check pump priming, delivery pressure, and pressure loss. P204A indicates a general electrical fault in that sensor circuit; it does not specify whether the signal is low, high, or intermittent.
Common causes
Most likely first.
- Sensor failed
- Harness fault
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
- Review freeze-frame data and live reductant pressure readings, along with related pump or dosing codes, to see when the circuit fault occurred.
- Inspect the reductant pressure sensor connector and harness for disconnection, corrosion, moisture, damaged insulation, or loose terminals.
- Check the sensor circuit wiring for opens, shorts, or poor contact using the manufacturer's wiring information.
- Test the pressure sensor signal and circuit against the manufacturer's specification to distinguish a sensor fault from a harness fault.
- If the electrical checks pass, verify that the reported pressure responds plausibly during the manufacturer's prescribed system checks.
Common fixes
- Reconnect or repair a loose, corroded, or damaged pressure sensor connector.
- Repair an open or short in the reductant pressure sensor wiring.
- Replace the reductant pressure sensor if it fails the manufacturer's electrical test.
Can I keep driving?
If the vehicle runs normally, short trips may be possible, but address the DEF or NOx warning promptly. The control module may not be able to verify pressure or dosing, which can increase emissions and lead to a no-start countdown or an inspection failure.
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.