Faulty reductant pressure sensor; Damaged or corroded wiring or connector at the sensor; Open or shorted circuit in the sensor signal or ground wire; Low DEF level or contaminated fluid causing pressure issues; Failed reductant pump or pressure regulator; Corroded or loose ground connection for the sensor circuit; Engine control module software or internal fault (rare)

DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P204C

P204CReductant Pressure Sensor Circuit Low

Q: What does code P204C mean?

Reductant Pressure Sensor Circuit Low. P204C means the engine control module has detected a low voltage signal from the reductant pressure sensor, which monitors the pressure of diesel exhaust fluid (DEF) in the selective catalytic reduction system. The sensor circuit is reporting a value below the expected range, so the computer cannot trust the pressure reading and sets this code.

P204C means the engine control module has detected a low voltage signal from the reductant pressure sensor, which monitors the pressure of diesel exhaust fluid (DEF) in the selective catalytic reduction system. The sensor circuit is reporting a value below the expected range, so the computer cannot trust the pressure reading and sets this code.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Reductant or DEF warning message displayed in the instrument cluster
Reduced engine power or limp mode in some vehicles
Poor fuel economy or increased DEF consumption
Difficulty starting or rough running in extreme cases
Exhaust fluid system-related messages on the driver information center

Common causes, most likely first

Faulty reductant pressure sensor
Damaged or corroded wiring or connector at the sensor
Open or shorted circuit in the sensor signal or ground wire
Low DEF level or contaminated fluid causing pressure issues
Failed reductant pump or pressure regulator
Corroded or loose ground connection for the sensor circuit
Engine control module software or internal fault (rare)

How to diagnose and fix it

Check DEF level and condition; top off or replace fluid if contaminated
Inspect sensor connector and wiring for damage, corrosion, or loose pins
Measure sensor reference voltage and ground at the connector with a multimeter
Test sensor signal voltage while monitoring live data on a scan tool
Check for technical service bulletins (TSBs) or software updates for the reductant system
Replace the reductant pressure sensor if voltage tests confirm it is faulty
Clear codes and perform a road test to verify the repair

Reductant pressure sensor low voltage traps

A low DEF level or diluted fluid can cause pressure readings to drop below threshold and set P204C even when the sensor itself is good.
Corroded pins in the sensor connector are a frequent cause that is often overlooked during visual inspection.
The code can be set by a failing reductant pump that cannot build enough pressure, not just by the sensor circuit.
Some vehicles require a relearn or adaptation reset after sensor replacement to avoid a repeat code.
P204C may appear alongside other reductant codes, so diagnosing the root cause requires checking all related faults.

Can I keep driving with P204C?

You can usually drive with P204C, but the vehicle may enter limp mode or limit speed if the fault worsens. Avoid long trips and heavy loads until the issue is diagnosed, and do not ignore the code because it can lead to more extensive emissions system damage.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.