Technical Description of OBD-II Trouble Code
The control circuit for Reductant Heater C 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. While the code is generic, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
If you have a stored code P20C4, it means that the powertrain control module (PCM) has detected excess voltage in the control circuit for the on-board reductant heater designated as “C.” If your vehicle has multiple reductant heaters, consult a vehicle-specific repair manual to determine which one is the C circuit 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 go a step further in reducing NOx. However, larger and more powerful diesel engines today 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 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 entire SCR system is monitored and controlled by either the PCM or a stand-alone controller that interacts with the PCM. The controller monitors the O2, NOx, and exhaust temperature sensors, among other inputs, to determine the appropriate time for DEF (reductant) 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 excess voltage on the control circuit for reductant heater C, it will store a code P20C4 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 P20C4 should be considered severe and addressed as quickly as possible. It could disable the SCR system and potentially cause catalyst damage if the conditions that led to the code being stored are not rectified promptly.
What are some of the symptoms of the code?
Symptoms of a P20C4 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code include:
What are some P20C4 troubleshooting steps?
To diagnose a code P20C4, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and vehicle-specific diagnostic information.
If you can find 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, it 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.
Next, 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 code may be more challenging at this time since it is intermittent. In such cases, the conditions that led to the code being stored may need to worsen before an accurate diagnosis can be made.
If the code is 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 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 correct, use the scanner to activate the reductant heater(s) and test the output control circuit voltage. Insufficient voltage may indicate a faulty or programmatically erroneous controller.
If the voltage output circuit is within parameters, use the DVOM to test the specific reductant heater element. If the heater fails to meet the manufacturer’s specifications, it is likely defective.
