P2039 Reductant Injection Air Pressure Sensor “A” Circuit Low

Technical Description of OBD-II Trouble Code

The circuit for the reductant injection air pressure sensor A is experiencing low voltage.

What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer), including BMW, Mercedes-Benz, Dodge, Sprinter, and others. The specific repair steps may vary depending on the year, make, model, and powertrain configuration.

A stored code P2039 indicates that the powertrain control module (PCM) has detected insufficient voltage in the circuit for the reductant injection air pressure sensor A. Consult a model-specific service manual to identify the “A” circuit.

The catalyst system is responsible for reducing all 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 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 before the diesel particulate filter, NOx trap, and/or catalytic converter using the reductant injection valve (solenoid). The precise timing of DEF injection increases the temperature of the filtration element, improving its efficiency. This extends the lifespan of the filtration element and reduces harmful exhaust emissions. The entire SCR system is monitored and controlled by the PCM or a stand-alone controller, which interacts with the PCM. The controller monitors O2, NOx, and exhaust temperature sensors, among other inputs, to determine the optimal time for DEF (reductant) injection. Accurate DEF injection is crucial for maintaining acceptable exhaust temperature and optimizing 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. The PCM also monitors one or more pressure sensors in the reductant supply system to detect any leaks.

If the PCM detects insufficient voltage in the circuit for the reductant injection air pressure sensor A, it will store a code P2039 and may illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be required for MIL illumination.

What is the severity of this DTC?

A stored code P2039 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 resolved promptly.

What are some of the symptoms of the code?

Symptoms of a P2039 trouble code may include:

What are some of the common causes of the code?

Possible causes for this code include:

What are some P2039 troubleshooting steps?

Ensure that the reductant/regeneration supply system is not losing pressure internally or externally. Activate the pump to create pressure and inspect the system for external leaks. Use a fuel pressure tester to manually monitor reductant system pressure. Check the supply pump and injector for any leaks, which must be repaired before continuing with the diagnosis.

To diagnose a code P2039, you will need 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’s year, make, model, engine size, stored codes, and exhibited symptoms. A TSB may provide helpful diagnostic information.

Use the scanner connected to the vehicle diagnostic connector to retrieve all stored codes and relevant freeze frame data. It is advisable to record this information before clearing the codes and then 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 underlying conditions may need to worsen before an accurate diagnosis can be made.

If the code is immediately reset, the next step in the diagnosis involves searching 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 the manufacturer’s 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 P2039 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.

Professional Service

We always recommend resolving this issue through mechanical means. However, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.