What it means
DEF pressure sensor reads below the lower plausibility limit — short to ground or sensor element shorted.
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 sends the SCR control module a signal representing DEF/AdBlue pressure. P204C sets when the signal falls below the expected electrical range, consistent with a signal pulled toward ground or a failed sensor element.
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
- Use freeze-frame data and live pressure-sensor data to confirm the signal is low and note whether it changes during pump priming.
- Inspect the sensor connector and harness for chafing, moisture, corrosion, bent pins, or a wire contacting ground.
- Check the sensor reference, ground, and signal circuits for shorts and poor connections using the vehicle wiring information and manufacturer's specification.
- With the sensor disconnected as directed by service information, determine whether the low signal remains in the harness or changes, which helps separate a wiring short from a sensor fault.
- Check actual DEF pressure and pump operation if the electrical signal tests normally, so a pressure-system problem is not mistaken for a low circuit signal.
- Repair the confirmed wiring or sensor fault, then verify the signal during a prime and road test.
Common fixes
- Clean and secure corroded or loose pressure-sensor connector terminals where permitted by service guidance.
- Repair chafed wiring or a signal wire shorted to ground.
- Replace the pressure sensor if its output remains below range with sound reference, ground, and wiring.
- Clear the fault and confirm the pressure signal responds during pump priming.
Can I keep driving?
A short drive may be possible if the vehicle behaves normally, but address the fault promptly and watch for SCR warnings or a countdown. Continued driving can leave emissions control impaired and may eventually 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.