Technical Description of OBD-II Trouble Code
Over Temperature Bank 2 – SCR NOx Prealyst
What does that signify?
This is a generic diagnostic trouble code (DTC) that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from Audi, Buick, Chevrolet, Ford, GMC, Mercedes-Benz, Subaru, Toyota, Volkswagen, and others. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
When a P20F1 code is stored in a diesel-powered, OBD-II equipped vehicle, it indicates that the powertrain control module has detected that the temperature for the selective catalyst reduction system (SCR) system exceeds the maximum allowable parameters. This code specifically pertains to the upstream temperature sensor for engine bank two. Bank two refers to the bank of the engine that contains the number two cylinder.
While modern, clean-burning diesel engines have numerous advantages over gasoline engines, especially in commercial truck applications, they also tend to produce higher levels of certain harmful exhaust gases, such as nitrogen oxide (NOx) ions.
Exhaust gas recirculation (EGR) systems help to significantly reduce NOx emissions, but many powerful diesel engines today cannot meet strict federal (U.S.) emission standards using the EGR system alone. This is why SCR systems were developed.
SCR systems inject diesel exhaust fluid (DEF) into the exhaust system in front of the catalytic converter or the NOx trap. The introduction of DEF raises the exhaust gas temperature and allows the catalyst element to function more efficiently. It prolongs the lifespan of the catalyst and reduces the release of NOx emissions into the atmosphere.
Temperature sensors are installed before and after the catalyst to monitor its temperature and efficiency. The entire SCR system is controlled by either the PCM or a stand-alone controller that interacts with the PCM. Regardless, the controller monitors the temperature sensors (among other inputs) to determine the appropriate time for DEF injection. Precise DEF injection is crucial for maintaining exhaust temperature within acceptable parameters and ensuring optimal NOx filtration.
If the PCM detects a prealyst temperature that exceeds the maximum allowable parameters, it will store a P20F1 code and may illuminate a malfunction indicator lamp.
What is the severity of this DTC?
Any stored SCR-related codes can result in the SCR system being disabled. The stored P20F1 code should be considered severe and addressed as soon as possible. Failure to rectify the code promptly could lead to catalyst damage.
What are some of the symptoms of the code?
Symptoms of a P20F1 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P20F1 troubleshooting steps?
If there are any other stored SCR or exhaust emissions codes, or exhaust temperature codes, they should be addressed before attempting to diagnose the stored P20F1 code.
Any prealyst exhaust leaks should be repaired before attempting a diagnosis for this type of code.
To diagnose a P20F1 code, you will need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer with a laser pointer, and vehicle-specific SCR system diagnostic information.
Locating 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, could provide helpful diagnostic information.
Begin the diagnosis by visually inspecting the SCR injection system, exhaust temperature sensors, NOx sensors, and oxygen (02) sensor harnesses and connectors. Any burnt or damaged wiring and connectors should be repaired or replaced before proceeding.
Next, locate the vehicle diagnostic connector and connect the scanner. Retrieve all stored codes and relevant freeze frame data, and record this information before clearing the codes. Then, test drive the vehicle until the PCM either 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 at this time. The conditions that contributed to the code being stored may need to worsen before a diagnosis can be made.
If the code is immediately reset, consult your source of vehicle information to obtain diagnostic flow charts, connector pinout charts, connector face views, as well as component testing procedures and specifications. This information will be necessary to complete the next steps of the diagnosis.
Observe the scanner data stream to compare exhaust (pre and post) temperature sensor data between engine banks. If any inconsistencies are detected, test the exhaust temperature sensors using the DVOM. Sensors that fail to comply with manufacturer specifications should be considered defective.
If all sensors and circuitry appear to be functioning as intended, suspect that the catalyst element is faulty or that the SCR system has failed.
