Technical Description of OBD-II Trouble Code
The control circuit for the reductant heater designated as D is open or not functioning properly.
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 of the vehicle.
A stored code P20C5 indicates that the powertrain control module (PCM) has detected no 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 the D designation 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 emissions. 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 led to the invention of selective catalytic reduction (SCR) systems.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust in front of 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 the release of harmful exhaust emissions into the atmosphere.
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 appropriate time for DEF injection. Precise DEF injection is crucial for maintaining exhaust temperature within acceptable parameters 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 no voltage on the control circuit for reductant heater D, a code P20C5 will be stored, and a malfunction indicator lamp may illuminate.
Reductant heaters may be located inside a DEF tank (example shown here):
What is the severity of this DTC?
A stored code P20C5 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 P20C5 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code include:
What are some P20C5 troubleshooting steps?
To diagnose a code P20C5, 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 engine size, stored code/s, and exhibited symptoms. This information can provide valuable diagnostic guidance.
Begin 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 contributed 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 to obtain diagnostic flow charts, connector pinout charts, connector face views, and component testing procedures and specifications. This information is essential 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 defective.
