Technical Description of OBD-II Trouble Code
The efficiency of the pre-catalyst for SCR NOx is below the threshold for Bank 1.
What does that mean?
This is a generic diagnostic trouble code (DTC) that applies to many OBD-II vehicles (1996-newer), including Audi, Buick, Chevrolet, Ford, GMC, Mercedes-Benz, Subaru, Toyota, Volkswagen, etc. The repair steps may vary depending on the year, make, model, and powertrain configuration.
If a P20EF code is stored in a diesel-powered, OBD-II equipped vehicle, it means that the powertrain control module has detected that the catalyst efficiency for a certain engine bank is below the threshold. This code specifically applies to the upstream catalytic converter (or NOx trap) for engine bank one, which is the bank that contains the number one cylinder.
While modern diesel engines have advantages over gasoline engines, they also tend to produce more harmful exhaust gases, particularly nitrogen oxide (NOx) ions.
Exhaust gas recirculation (EGR) systems help reduce NOx, but many powerful diesel engines cannot meet strict emission standards using EGR alone. This is why SCR systems were developed.
SCR systems inject diesel exhaust fluid (DEF) into the exhaust in front of the catalytic converter or NOx trap. DEF raises exhaust gas temperature, allowing the catalyst element to perform more efficiently, prolonging its lifespan and reducing NOx emissions.
Oxygen (O2) sensors, NOx sensors, and temperature sensors are placed before and after the catalyst to monitor its temperature and efficiency. The entire SCR system is controlled by the PCM or a stand-alone controller that interacts with the PCM. The controller monitors the O2, NOx, and temperature sensors, among other inputs, to determine the appropriate time for DEF injection. Precise DEF injection is necessary to maintain exhaust temperature within acceptable parameters and optimize NOx filtration.
If the PCM detects insufficient catalyst efficiency, it stores a P20EF code and may illuminate the malfunction indicator lamp.
What is the severity of this DTC?
Any stored SCR-related codes may disable the SCR system. The stored P20EF code should be considered severe and addressed promptly to prevent catalyst damage.
What are some of the symptoms of the code?
Symptoms of a P20EF trouble code may include:
What are some of the common causes of the code?
Possible causes for this code include:
What are some P20EF troubleshooting steps?
If other SCR or exhaust emissions codes or exhaust temperature codes are stored, address those first before diagnosing the stored P20EF code.
Repair any pre-catalyst exhaust leaks before attempting to diagnose this code.
To diagnose a P20EF code, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer with a laser pointer, and vehicle-specific SCR system diagnostic information.
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. This TSB may provide helpful diagnostic information.
Begin the diagnosis with a visual inspection of the SCR injection system, exhaust temperature sensors, NOx sensors, and oxygen (O2) sensor harnesses and connectors. Repair or replace any burnt or damaged wiring or connectors before proceeding.
Next, locate the vehicle diagnostic connector and connect the scanner. Retrieve all stored codes and 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 difficult to diagnose at this time. The conditions that led 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, component testing procedures, and specifications. This information is necessary to complete the next steps of the diagnosis.
Observe the scanner data stream to compare exhaust (pre and post) O2, NOx, and temperature sensor data between engine banks. If inconsistencies are detected, test the respective 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 correctly, suspect a faulty catalyst element or a failed SCR system.
