Technical Description of OBD-II Trouble Code
Circuit for Reductant Pressure Sensor
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Dodge, Ram, VW, Audi, GMC, BMW, Mercedes-Benz, Ford, Chevrolet, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
A stored code P204A indicates that the powertrain control module (PCM) has detected an abnormal voltage level in the reductant pressure sensor circuit. This code is observed exclusively in vehicles with a clean burning diesel engine.
The catalyst system is responsible for reducing almost all exhaust emissions, although certain applications also have a NOx trap.
Exhaust gas recirculation (EGR) systems go a step further in reducing NOx. However, larger and more powerful diesel engines of today 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 developed.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust system before 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 function more efficiently. This improves the longevity of the filtration element and reduces the release of harmful exhaust emissions 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 the O2, NOx, and exhaust temperature sensors, among other inputs, to determine the optimal time for DEF (reductant) injection. Precise DEF injection is crucial for maintaining exhaust temperature within acceptable limits and optimizing 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. It also monitors one or more pressure sensors in the reductant supply system to detect any leaks.
If the PCM detects an abnormal voltage level in the reductant temperature sensor circuit, it will store a code P204A and may illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be necessary for MIL illumination.
Example of a DEF reductant tank:
What is the severity of this DTC?
A stored code P204A should be considered severe and addressed as soon as possible. It could disable the SCR system, and failure to rectify the conditions that led to the code being stored in a timely manner could result in catalyst damage.
What are some of the symptoms of the code?
Symptoms of a P204A trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P204A troubleshooting steps?
To diagnose a code P204A, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
Refer to 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 codes, and exhibited symptoms. A relevant TSB may provide helpful diagnostic information.
Use the scanner connected to the vehicle diagnostic connector to retrieve all stored codes and freeze frame data. It is recommended to write down this information before clearing the codes and then test-drive the vehicle until the PCM enters readiness mode or the code is reset.
If the PCM enters readiness mode, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the code being stored may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step in the diagnosis involves consulting 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. Any components that do not test within the maximum allowable parameters should be considered defective.
Step 2
If the actual reductant system pressure is within specifications, the code P204A persists, and the sensor in question is functioning properly, use the DVOM to test the input and output signal circuits between the pressure sensor and the PCM/SCR controller. Disconnect all controllers before conducting the DVOM tests.
