Technical Description of OBD-II Trouble Code
The circuit for the Exhaust Aftertreatment Fuel Pressure Sensor is experiencing low voltage.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer), including vehicles from Mercedes Benz, Mitsubishi, and others. The exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your diesel powered vehicle has a stored code P20DF, it means that the powertrain control module (PCM) has detected insufficient voltage in the fuel pressure sensor 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 include 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 consists of 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. An EAS fuel pressure sensor allows the controller to monitor system pressure when the supply pump is activated. A malfunction has been detected in this circuit when a code P20DF is stored.
Among other functions, exhaust aftertreatment systems (EAS) are responsible for injecting reductant compound/diesel exhaust fluid (DEF) into the exhaust system in front of the diesel particulate filter, NOx trap, and/or the catalytic converter. Precisely timed DEF injections increase the temperature of the filtration elements and improve their efficiency. Introducing DEF into the catalyst system promotes longevity of the filtration elements and reduces harmful exhaust emissions.
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 injection.
If the PCM detects insufficient voltage on the circuit for the EAS fuel pressure sensor, a code P20DF will be stored and a malfunction indicator lamp may illuminate.
What is the severity of this DTC?
A stored code P20DF should be considered severe and addressed as quickly as possible. The EAS system may be damaged as a result of the conditions that led to the code being stored.
What are some of the symptoms of the code?
Symptoms of a P20DF trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P20DF troubleshooting steps?
To diagnose a code P20DF, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
It may be helpful 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.
Start the diagnosis by visually inspecting the EAS wiring harnesses and connectors. Repair or replace any burnt or damaged wiring and connectors before proceeding.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and relevant freeze frame data. Consider writing 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, the code is intermittent and may be more difficult to diagnose. In this case, 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 of the diagnosis will require searching for vehicle information to obtain diagnostic flow charts, connector pin-out charts, connector face views, and 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 all system fuses are intact, use the DVOM to test the EAS fuel pressure sensor and sensor circuits. If any of these components fail to meet the manufacturer’s specifications, suspect that they have failed.
