Technical Description of OBD-II Trouble Code
The control circuit for the reductant heater designated as D is experiencing high voltage.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many diesel OBD-II vehicles (1996-newer), including vehicles from Ford, Mercedes Benz, Sprinter, Audi, Ram, Chevrolet, Dodge, BMW, GMC, etc. The exact repair steps may vary depending on the year, make, model, and powertrain configuration.
A stored code P20C8 indicates that the powertrain control module (PCM) has detected excess voltage in the control circuit for the on-board reductant heater designated as D. If multiple reductant heaters are in use, consult a vehicle-specific repair manual to determine which circuit corresponds to D for your application.
The catalyst system is responsible for reducing most of the other exhaust emissions, although some applications also have a NOx trap.
Exhaust gas recirculation (EGR) systems further reduce NOx. However, larger and more powerful diesel engines cannot meet strict federal (U.S.) emission standards with just the EGR, diesel particulate filter/catalytic converter, and NOx trap. This is why selective catalytic reduction (SCR) systems were invented.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust before the diesel particulate filter and/or catalytic converter. The precise timing of DEF injection increases the temperature of the filtration element, allowing it to perform more efficiently. This extends the lifespan of the filtration element and reduces harmful exhaust emissions.
The PCM or a stand-alone controller monitors and controls the entire SCR system. The controller uses inputs from O2, NOx, and exhaust temperature sensors, among others, to determine the optimal time for DEF injection. Precise DEF injection is crucial for maintaining exhaust temperature within acceptable limits and optimizing pollutant filtration.
Reductant heaters are used to prevent diesel exhaust fluid from freezing in extreme temperatures. These heaters are typically located in the DEF reservoir and/or the reductant injector supply hose(s).
If the PCM detects excess voltage on the control circuit for reductant heater D, it will store a code P20C8 and may illuminate the malfunction indicator lamp.
Reductant heaters may be located inside a DEF tank (example shown here):
What is the severity of this DTC?
A stored code P20C8 should be considered severe and addressed as quickly as possible. It could disable the SCR system and potentially cause catalyst damage if the underlying conditions are not rectified promptly.
What are some of the symptoms of the code?
Symptoms of a P20C8 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code include:
What are some P20C8 troubleshooting steps?
To diagnose a code P20C8, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and vehicle-specific diagnostic information.
If available, consult a technical service bulletin (TSB) that matches the vehicle’s year, make, and model, as well as the stored code/s and exhibited symptoms. This can provide helpful diagnostic information.
Start the diagnosis by visually inspecting the reductant heater system harnesses and connectors. Repair or replace any burnt or damaged wiring and connectors before proceeding.
Connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and relevant freeze frame data. Write down this information before clearing the codes and test driving the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode, diagnosing the intermittent code may be more challenging at this time. It may be necessary for the conditions that led to the code being stored to worsen before an accurate diagnosis can be made.
If the code is reset, refer to your source of vehicle information for diagnostic flow charts, connector pinout charts, connector face views, and component testing procedures and specifications. This information is necessary for the next step of the diagnosis.
Use the DVOM to test the power supply to the SCR control system. Test the fuses with the circuit loaded to avoid misdiagnosis. If the power and ground circuits are functioning correctly, use the scanner to activate the reductant heater(s) and test the output control circuit voltage. Insufficient voltage may indicate a faulty or improperly programmed controller.
If the voltage output circuit is within parameters, use the DVOM to test the specific reductant heater element. If the heater does not meet the manufacturer’s specifications, it is likely faulty.
