Technical Description of OBD-II Trouble Code
Performance Issue in Exhaust Aftertreatment Fuel Supply Control Circuit
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 VW, Audi, Mercedes Benz, Ford, Mitsubishi, and others. While it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
If your diesel-powered vehicle has stored a code P20D8, it means that the powertrain control module (PCM) has detected abnormal voltage outside the expected range in the fuel supply control circuit for the exhaust aftertreatment system.
The exhaust aftertreatment system, also known as the selective catalyst reduction system, is used to enhance the capabilities of the exhaust catalyst system. It may consist of components such as the diesel oxidation catalyst, diesel particulate filter, reductant injection system, ammonia slip catalyst, and a nitrogen oxide (NOx) trap.
The reductant injection system typically includes at least one reductant injector, a reductant fuel storage tank, and high-pressure reductant fuel lines. A high-pressure electronic pump is usually located in the tank or fuel supply line. It is within the circuit that controls this supply pump that a malfunction has been detected when a P20D8 code is stored.
Among other functions, exhaust aftertreatment systems (EAS) are responsible for injecting reductant compound/diesel exhaust fluid (DEF) into the exhaust system ahead of the diesel particulate filter, NOx trap, and/or the catalytic converter using an automated fluid storage and injection system. Precisely timed DEF injections raise the temperature of the various filtration elements, allowing them to operate more efficiently. Introducing DEF into the catalyst system promotes the longevity of filtration elements and reduces the release of harmful exhaust emissions into the atmosphere.
The EAS and catalyst systems are monitored and controlled by either the PCM or a stand-alone controller that interacts with the PCM. The controller monitors the reductant fluid injection system pressure, O2, NOx, and exhaust temperature sensors, among other inputs, to determine the appropriate time for DEF (reductant) injection.
If the PCM detects abnormal voltage outside the expected range in the control circuit for the EAS fuel pump, a P20D8 code will be stored and a malfunction indicator lamp may illuminate.
What is the severity of this DTC?
A stored P20D8 code should be considered severe and addressed promptly. The EAS system may suffer damage due to the conditions that led to the storage of the P20D8 code.
What are some of the symptoms of the code?
Symptoms of a P20D8 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P20D8 troubleshooting steps?
To diagnose a P20D8 code, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific 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.
I recommend starting the diagnosis with a visual inspection of the EAS wiring harnesses and connectors. Any burnt or damaged wiring or connectors should be repaired or replaced before proceeding.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and relevant freeze frame data. It may be helpful to write down this information before clearing the codes. Test drive the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this point, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step of the diagnosis will require searching for vehicle information to obtain diagnostic flow charts, connector pin-out charts, connector face views, as well as component testing procedures and specifications.
Use the DVOM to test (voltage drop) all EAS fuel supply system grounds. Then, test the power supply to the EAS control system. Test fuses with the circuit loaded to avoid misdiagnosis. If the appropriate power (battery voltage) and ground circuits are found, use the scanner to activate the EAS fuel pump and test the output control circuit voltage. If the voltage is insufficient, test the EAS fuel pump relay. If no input voltage is detected, suspect a faulty or incorrectly programmed controller. If input voltage is present on the relay but output voltage is not detected, suspect a faulty relay.
If the EAS power supply voltage output circuit is within parameters, use the DVOM to test the EAS fuel pressure sensor and fuel pump. If any of these components fail to meet the manufacturer’s specifications, suspect that they have failed.
