P204D: Reductant Pressure Sensor Circuit High

What it means

DEF pressure sensor reads above the upper plausibility limit — open signal or short to B+. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.

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 tells the SCR control module how much pressure the DEF/AdBlue pump is producing. P204D sets when the signal is above its expected electrical range, often because the signal circuit is open or pulled toward battery voltage rather than reporting pressure correctly.

Common causes

Most likely first.

1. Open signal line
2. Connector pins corroded - high contact resistance
3. 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

1. Capture freeze-frame data and observe live pressure-sensor data during a pump prime to confirm the signal is high or implausible.
2. Inspect the pressure sensor connector for water, corrosion, backed-out pins, and poor terminal fit, then examine the harness for chafing or contact with a power feed.
3. Check the sensor signal wire for an open circuit and for a short to battery voltage, following the wiring diagram and manufacturer's specification.
4. Test the sensor reference and ground circuits and verify continuity from the sensor connector to the control module before condemning either component.
5. If the signal wiring is intact, assess the sensor output and actual DEF pressure during priming to distinguish an electrical fault from a sensor fault.
6. After repair, verify that the signal follows pressure changes and that the code does not return.

Common fixes

• Restore terminal fit or repair water-damaged or corroded connector contacts.
• Repair an open signal wire or a harness short to battery voltage.
• Replace the pressure sensor if its output remains high with correct wiring and supply circuits.
• Clear the code and confirm normal pressure-sensor response during priming.

Can I keep driving?

If the vehicle runs normally, limit driving to what is needed to get the fault diagnosed and heed any SCR warning or countdown. An incorrect pressure signal can disable proper reductant dosing, affect emissions, and eventually lead to a no-start restriction.