Technical Description of OBD-II Trouble Code
The control circuit for Exhaust Aftertreatment Fuel Injector A is open
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer). This includes vehicles from Mercedes Benz, Sprinter, GMC, Chevrolet, Ford, etc. Although it is a generic code, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your diesel-powered vehicle has stored a code P20CB, it means that the powertrain control module (PCM) has detected no voltage in the control circuit for the exhaust aftertreatment fuel injector, designated A. The designation A indicates that multiple reductant injectors are in use.
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 one or more of these components: diesel oxidation catalyst, diesel particulate filter, reductant injection system, ammonia slip catalyst, and a nitrogen oxide (NOx) trap.
Among other functions, exhaust aftertreatment systems (EAS) are responsible for injecting reductant compound/diesel exhaust fluid (DEF) into the exhaust in front of the diesel particulate filter, NOx trap, and/or the catalytic converter via an automated fluid storage and injection system. Introducing DEF into the catalyst system promotes the longevity of the filtration element and reduces harmful exhaust emissions released 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 O2, NOx, and exhaust temperature sensors (among other inputs) to determine the appropriate time for DEF (reductant) injection. DEF injection must be performed at the correct moment and in precise measure to maintain an exhaust temperature within acceptable parameters and optimize pollutant filtration.
If the PCM detects no voltage on the control circuit for the EAS fuel injector, a code P20CB will be stored and a malfunction indicator lamp may illuminate.
What is the severity of this DTC?
A stored code P20CB 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 P20CB being stored.
What are some of the symptoms of the code?
Symptoms of a P20CB trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P20CB troubleshooting steps?
To diagnose a code P20CB, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information.
Locating a technical service bulletin (TSB) that matches the vehicle 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 and 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 is advisable 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 code being stored may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step of your diagnosis will require searching your vehicle information source to obtain diagnostic flow charts, connector pin-out charts, connector face views, as well as component testing procedures and specifications. You can also use your vehicle information source to pinpoint the exact location of EAS fuel injector A.
Use the DVOM to test (voltage drop) 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 injector (solenoid) and test the output control circuit voltage. If the voltage is insufficient, suspect that the controller is faulty or has experienced a programming error.
If the voltage output circuit is within parameters, use the DVOM to test the EAS fuel injector in question. If the injector fails to meet the manufacturer’s specifications, suspect that it has malfunctioned.
