Technical Description of OBD-II Trouble Code
The circuit for the Reductant Injection Air Pressure Sensor B is registering a high voltage, resulting in this diagnostic trouble code (DTC). This code is a generic powertrain code that applies to various OBD-II vehicles manufactured from 1996 onwards, including BMW, Mercedes-Benz, Dodge, Sprinter, and others. The specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
A stored code P209D indicates that the powertrain control module (PCM) has detected excessive voltage in the reductant injection air pressure sensor B circuit. The “B” designation refers to a particular sensor in a system that may have multiple pressure sensors. To determine which circuit corresponds to “B,” consult a model-specific service manual.
The catalyst system is responsible for reducing exhaust emissions, including NOx. Some applications also feature a NOx trap. Exhaust gas recirculation (EGR) systems further reduce NOx emissions. However, larger and more powerful diesel engines cannot meet strict U.S. emission standards with just the EGR, diesel particulate filter/catalytic converter, and NOx trap. This led to the development of selective catalytic reduction (SCR) systems.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust system before the diesel particulate filter, NOx trap, and/or catalytic converter. The precise injection of DEF enhances the performance and longevity of the filtration element, reducing harmful exhaust emissions. The PCM or a stand-alone controller monitors and controls the entire SCR system, including O2, NOx, and exhaust temperature sensors, to optimize pollutant filtration.
The reductant/regeneration supply pump pressurizes the DEF in the liquid reductant system when needed. The PCM monitors the supply pump voltage for consistent fluctuation and load percentage. Additionally, one or more pressure sensors in the reductant supply system are monitored for leaks.
If the PCM detects excessive voltage in the reductant injection air pressure sensor B circuit, it will store a code P209D and may illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be required for MIL illumination.
Severity of this DTC
A stored code P209D should be considered severe and addressed promptly to prevent potential disabling of the SCR system and catalyst damage.
Symptoms of the code
Symptoms associated with a P209D trouble code may include:
Common causes of the code
Possible causes for this code include:
Troubleshooting steps for P209D
To diagnose a code P209D, ensure that the reductant/regeneration supply system is not losing pressure internally or externally. Check for external leaks by activating the pump to create pressure and inspecting the system. Use a fuel pressure tester to monitor reductant system pressure manually. If leaks are found, they must be repaired before proceeding with further diagnosis.
Diagnosing a code P209D requires a diagnostic scanner, a digital volt/ohmmeter (DVOM), and vehicle-specific diagnostic information. Consult your vehicle information source to find a technical service bulletin (TSB) that matches the vehicle details and symptoms. Retrieve all stored codes and freeze frame data using the scanner before clearing the codes. Test-drive the vehicle until the PCM enters readiness mode or the code is reset.
If the PCM enters readiness mode, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions contributing to the code being stored may need to worsen for an accurate diagnosis. If the code is immediately reset, refer to your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 1
Use the DVOM to test the reductant injection system pressure sensors according to manufacturer specifications. Consider components that do not test within the maximum allowable parameters as defective.
Step 2
If the reductant injection pressure is within specifications, the code P209D persists, and the sensor in question is functioning correctly, use the DVOM to test the input and output signal circuits between the sensors and the PCM/SCR controller. Disconnect all controllers before conducting the DVOM tests.
