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Technical Description of OBD-II Trouble Code
The exhaust pressure control valve “A” is operating at a high level.
What does this mean?
This generic diagnostic trouble code is typically found in diesel engines, including certain Ford Powerstroke, Dodge Cummins, Mercedes, Nissan, and VW vehicles. It can also apply to trucks with diesel engines and dealer-installed exhaust brakes.
An exhaust pressure control valve is placed in the exhaust stream after the exhaust manifold to generate back pressure and heat. This back pressure and heat can assist in cold start warm-up and slow down the engine and vehicle by opposing cylinder pressure coming from the engine cylinders out of the exhaust. This is particularly useful during towing operations.
This code specifically relates to the exhaust pressure sensor not matching intake manifold pressure or ambient air pressure during normal driving. It can be caused by a mechanical or electrical circuit fault, depending on the vehicle manufacturer.
Troubleshooting steps may vary depending on the manufacturer, type of exhaust back pressure control, and wire colors to the control solenoid.
Symptoms
Symptoms of a P0478 engine code may include:
Potential Causes
The typical causes for this code to set are:
Diagnostic and Repair Procedures
A good starting point is to search for a technical service bulletin (TSB) specific to your vehicle. The vehicle manufacturer may have a PCM flash/reprogram to address this issue, so it’s worth checking before going down the wrong path. PCM stands for powertrain control module.
Next, locate the exhaust pressure sensor on your vehicle and visually inspect the connectors and wiring. Look for any signs of damage such as chafing, rubbing, bare wires, burn spots, or melted plastic. Disconnect the connectors and inspect the terminals inside for corrosion, burning, or unusual discoloration. If cleaning is necessary, use electrical contact cleaner or rubbing alcohol and a plastic bristle brush. After cleaning, let the connectors air dry and apply dielectric silicone compound to the terminals.
If your vehicle is equipped, remove the sensing tube that connects the exhaust backpressure sensor to the exhaust manifold and check if you can blow through it. If it is blocked, it could be a possible cause for this code.
If you have a scan tool, clear the diagnostic trouble codes from memory and see if this code returns. If it does not, then the issue was likely with the connections.
If the code does return, you will need to verify proper turbocharger boost operation. Use a scan tool that can read turbocharger boost pressure or monitor intake manifold pressure. Note the pressure at Key On with the Engine Off, then start the engine, drive at a safe speed, and momentarily accelerate the engine to wide open throttle without exceeding 2500-3000 RPMs. You should observe a change in pressure of at least 18 PSI, possibly more depending on the vehicle manufacturer and year.
If this test passes or if you were unable to check turbocharger boost, you will need to test the sensor and its associated circuits. Typically, there are 3 wires at the exhaust pressure sensor.
Disconnect the harness going to the exhaust pressure sensor and use a Digital Volt Ohm Meter (DVOM) to test the 5V power supply circuit. Ensure that it is receiving the correct voltage (5V) by connecting the red lead to the power supply circuit and the black lead to a good ground. If there is 12 volts instead of 5 volts, repair the wiring or consider a possible issue with the PCM.
If the power supply circuit is functioning correctly, use the DVOM to check the signal circuit. Ensure that it is receiving 5V by connecting the red lead to the signal circuit and the black lead to a good ground. If there is no 5 volts or if there is 12 volts, repair the wiring or consider a possible issue with the PCM.
Next, check the ground circuit at the exhaust pressure sensor. Connect a test light to the positive terminal of a 12V battery and touch the other end to the ground circuit. If the test light does not illuminate, there is a problem with the ground circuit. If it does illuminate, wiggle the wiring harness going to the sensor to check for intermittent connections.
If all previous tests have passed and you continue to receive a P0478 code, it is likely that the exhaust pressure sensor has failed. However, a stuck closed exhaust backpressure valve or a failed PCM cannot be ruled out until the sensor is replaced.
Professional Service
While we always recommend seeking professional mechanical assistance to resolve this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to be addressed. If you are interested in this option, please upload your ECU file to our portal.
