Technical Description of OBD-II Trouble Code
The circuit for the reductant pressure sensor is experiencing low voltage.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer), including vehicles from Dodge, Ram, VW, Audi, GMC, BMW, Mercedes-Benz, Ford, Chevrolet, etc. The exact repair steps may vary depending on the year, make, model, and powertrain configuration.
A stored code P204C indicates that the powertrain control module (PCM) has detected insufficient voltage in the reductant pressure sensor circuit. This code is found exclusively in vehicles with clean burning diesel engines.
The catalyst system is responsible for reducing all exhaust emissions, although some applications also have a NOx trap.
Exhaust gas recirculation (EGR) systems further reduce NOx. However, larger and more powerful diesel engines cannot meet strict federal (U.S.) emission standards with just the EGR, diesel particulate filter/catalytic converter, and NOx trap. This is why selective catalytic reduction (SCR) systems were invented.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust in front of the diesel particulate filter, NOx trap, and/or catalytic converter using the reductant injection valve (solenoid). The precise timing of DEF injection increases the temperature of the filtration element, allowing it to perform more efficiently. This improves filtration element longevity and reduces harmful exhaust emissions released into the atmosphere. The entire SCR system is monitored and controlled by either the PCM or a stand-alone controller, which interacts with the PCM. The controller monitors O2, NOx, and exhaust temperature sensors, among other inputs, to determine the appropriate time for DEF (reductant) injection. Precise DEF injection is necessary to maintain exhaust temperature within acceptable parameters and optimize pollutant filtration.
The reductant/regeneration supply pump pressurizes the DEF in the liquid reductant system for use when required. The PCM monitors the supply pump voltage for consistent fluctuation and load percentage. The PCM also monitors one or more pressure sensors in the reductant supply system to detect any leaks.
If the PCM detects voltage in the reductant temperature sensor circuit that is lower than acceptable parameters, it will store a code P204C and may illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be required for MIL illumination.
Example of a DEF reductant tank:
What is the severity of this DTC?
A stored code P204C should be considered severe and addressed as quickly as possible. It could disable the SCR system, and catalyst damage may occur if the conditions contributing to the stored code are not rectified promptly.
What are some of the symptoms of the code?
Symptoms of a P204C trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P204C troubleshooting steps?
To diagnose a code P204C, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information.
Consult your vehicle information source to find a technical service bulletin (TSB) that matches the vehicle’s year, make, and model, as well as the engine size, stored code/s, and exhibited symptoms. A TSB may provide helpful diagnostic information.
Use the scanner connected to the vehicle diagnostic connector to retrieve all stored codes and relevant freeze frame data. It is advisable to write down this information before clearing the codes and then test-drive the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this time, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions contributing to the stored code may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step in your diagnosis will involve searching your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 1
Use the DVOM to test the reductant pressure sensor according to the manufacturer’s specifications. Consider components that do not test within the maximum allowable parameters as defective.
Step 2
If the actual reductant system pressure is within specifications, the code P204C persists, and the sensor in question is functioning correctly, use the DVOM to test the input and output signal circuits between the pressure sensor and the PCM/SCR controller. Disconnect all controllers before using the DVOM for testing.
