Technical Description of OBD-II Trouble Code
Range/Performance Issue with Reductant Pressure Sensor Circuit
What does this mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to various OBD-II vehicles (1996-newer), including but not limited to Dodge, Ram, VW, Audi, GMC, BMW, Mercedes-Benz, Ford, Chevrolet, etc. The specific repair steps may vary depending on the year, make, model, and powertrain configuration.
A stored code P204B indicates that the powertrain control module (PCM) has detected abnormal 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 almost all exhaust emissions, although some applications also have a NOx trap.
Exhaust gas recirculation (EGR) systems further reduce NOx emissions. However, larger and more powerful diesel engines cannot meet strict U.S. emission standards with just the EGR, diesel particulate filter/catalytic converter, and NOx trap. This led to the development of selective catalytic reduction (SCR) systems.
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. The precise injection of DEF raises the temperature of the filtration element, improving its efficiency. This extends the lifespan of the filtration element and reduces harmful exhaust emissions. The entire SCR system is monitored and controlled by the PCM or a stand-alone controller. The controller monitors O2, NOx, and exhaust temperature sensors, among other inputs, to determine the optimal time for DEF injection. Accurate DEF injection is crucial for maintaining acceptable exhaust temperature and optimizing pollutant filtration.
The reductant/regeneration supply pump pressurizes the DEF in the liquid reductant system when needed. 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 abnormal voltage in the reductant temperature sensor circuit, it will store a code P204B and may illuminate the malfunction indicator lamp (MIL). The MIL may require multiple ignition cycles with a failure to illuminate.
Example of a DEF reductant tank:
How severe is this DTC?
A stored code P204B should be considered severe and addressed promptly, as it could disable the SCR system. Failure to rectify the conditions that led to the code being stored could result in catalyst damage.
What are some symptoms of the code?
Symptoms of a P204B trouble code may include:
What are some common causes of the code?
Possible causes for this code include:
What are some troubleshooting steps for P204B?
To diagnose a code P204B, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and vehicle-specific diagnostic information.
Refer to your vehicle information source to find a technical service bulletin (TSB) that matches the vehicle’s year, make, model, 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. 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 P204B 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 conducting the DVOM tests.
