Technical Description of OBD-II Trouble Code
The Exhaust Pressure Control Valve “A” Intermittent is the cause of this trouble code.
What does that signify?
This particular diagnostic trouble code is commonly found in diesel engines, specifically in certain Ford, Dodge, Mercedes, Nissan, and VW vehicles. It can also be applicable 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. Its purpose is to generate heat in the form of back pressure in the exhaust. This heat and back pressure 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 trouble code specifically relates to the incoming signal from the exhaust pressure sensor not matching the intake manifold pressure or ambient air pressure during normal driving. The cause can be either a mechanical or electrical circuit fault, depending on the vehicle manufacturer.
The troubleshooting steps may vary depending on the manufacturer, type of exhaust back pressure control, and wire colors to the control solenoid.
Indications
The symptoms of an engine code P0479 may include:
Possible Causes
The typical causes for this code to be 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 is worth checking before going down the wrong path. PCM stands for powertrain control module.
Next, locate the Exhaust Pressure Sensor on your vehicle. Once found, disconnect the tube that connects the sensor to the exhaust manifold. Try blowing through it. If you are unable to, use a small piece of wire to dislodge the trapped carbon inside, which is causing the fault code.
If the tube is clear and free, visually inspect the connectors and wiring. Look for any chafing, rubbing, bare wires, burn spots, or melted plastic. Separate the connectors and carefully examine the terminals inside. Check if they appear corroded, burnt, or possibly green in color instead of the normal metal color. If needed, use Electrical Contact cleaner or rubbing alcohol and a light plastic bristle brush to clean the terminals. After cleaning, let them air dry and apply some dielectric silicone compound where the terminals come into contact.
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 problem was most likely the connections.
If the code does return, you will need to verify the proper operation of the turbocharger boost. You will need a scan tool that can read turbocharger boost pressure. Alternatively, you can monitor the intake manifold pressure, as it provides similar information. Take note of the pressure at Key On, but with the Engine Off. Then start the engine, drive the vehicle at a safe speed, and momentarily accelerate the engine to wide open throttle, making sure the engine RPM does not exceed 2500-3000 RPMs. You should observe a change of at least 18 PSI, possibly more depending on the vehicle manufacturer and year.
If this test is successful 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. Use a Digital Volt Ohm Meter (DVOM) to test the 5V power supply circuit to ensure it is receiving power (Red lead to the 5V power supply circuit, black lead to a good ground). If there is 12 volts instead of 5 volts, repair the wiring from the PCM to the sensor for a short to 12 volts or possibly a faulty PCM.
If that is okay, use a DVOM to check if there is 5V on the Exhaust Pressure Sensor signal circuit (Red lead to the sensor signal circuit, black lead to a good ground). If there is no 5 volts or if you measure 12 volts, repair the wiring from the PCM to the sensor or consider a possible faulty PCM.
If that is okay, check if there is a good ground at the Exhaust Pressure sensor. Connect a test light to the positive terminal of a 12V battery and touch the other end of the test light to the ground circuit going to the Exhaust Pressure Sensor circuit ground. If the test light does not illuminate, there is a problem with the circuit. If it does illuminate, wiggle the wiring harness going to each terminal to check for intermittent connections.
If all the tests have passed so far and you continue to receive a P0479 code, try wiggling the sensor wiring harness while monitoring the scan tool to see if the code returns. If it does, this likely indicates an intermittent connection in the harness. If not, it is likely a failed Exhaust Pressure sensor, although a stuck closed Exhaust Pressure Control Valve or a failed PCM cannot be ruled out until the sensor is replaced.
