What does it mean?
This generic powertrain/engine diagnostic code generally applies to diesel engines, including but not limited to certain Ford, Dodge, Mercedes, Nissan, and VW vehicles.
This code may also apply to trucks equipped with diesel engines and dealer-installed exhaust brakes.
A valve is placed in the exhaust stream after the exhaust manifold to generate heat in the form of backpressure in the exhaust. This heat and/or backpressure can be used to assist with cold starts. It can also be used to oppose cylinder pressure from the engine’s cylinders to the exhaust, thereby slowing the engine and the vehicle with it. This is particularly useful during towing operations.
This code is strictly concerned with the incoming signal from the exhaust pressure sensor that does not match the intake manifold pressure or ambient air pressure during normal driving. It could be a mechanical or electrical circuit fault, depending on the vehicle manufacturer.
Troubleshooting steps may vary by manufacturer, type of exhaust backpressure control, and the color of the control solenoid wires.
Symptoms
Symptoms of a P0479 engine code may include:
Malfunction Indicator Light (MIL) on
Lack of power
Absence of engine braking
Longer than normal cold engine warm-up time
Potential Causes
Typically, the causes for this code to set are:
Blockage in the exhaust manifold tube to the pressure sensor
Exhaust system/air intake/supercharger air leaks
Intermittent open in the exhaust pressure sensor ground circuit
Intermittent open in the signal circuit between the exhaust pressure sensor and the PCM (Powertrain Control Module)
Intermittent short to voltage in the signal circuit to the exhaust pressure sensor
Exhaust gas pressure sensor
Possibly turbocharger overboost
Possibly the PCM has failed (highly unlikely)
Diagnostic and Repair Procedures
A good starting point is always to search for a Technical Service Bulletin (TSB) for your specific vehicle. The vehicle manufacturer may have a PCM flash/reprogram to cover this issue, and it’s worth checking before discovering you’ve taken a long/wrong path. PCM = Powertrain Control Module.
Next, locate the exhaust pressure sensor on your specific vehicle. Once located, disconnect the tube connecting the sensor to the exhaust manifold. Try blowing through it. If you cannot, try running a small piece of wire through it to dislodge the carbon trapped inside, causing the error code you’re experiencing.
If the tube is clear and free, visually inspect the connectors and wiring. Look for chafing, rubbing, bare wires, burns, or melted plastic. Separate the connectors and carefully inspect the terminals (the metal parts) inside the connectors. See if they look corroded, burnt, or perhaps greenish compared to the normal metal color you’re probably used to seeing. You can get electrical contact cleaner at any auto parts store if terminal cleaning is needed. If that’s not possible, find 91% rubbing alcohol and a light plastic-bristled brush to clean them. Then let them air dry, get some dielectric silicone compound (same as used for light bulb sockets and spark plug wires), and apply it where the terminals make contact.
If you have a scan tool, clear the diagnostic codes from memory and see if this code returns. If not, the connections were likely your problem.
If the code returns, you’ll need to check the proper operation of the turbocharger overboost. You’ll need a scan tool capable of reading the turbocharger boost pressure. You may need to monitor the intake manifold pressure, as this will provide the same information. Note the pressure at Key On, but with the engine off. After that, start the engine, drive the vehicle to a safe speed, then momentarily accelerate the engine to full throttle, ensuring the engine speed does not exceed 2500-3000 RPM. You should note a change of at least 18 PSI, perhaps more depending on the vehicle manufacturer and the vehicle’s model year.
If this test passed, or if you were unable to check the turbocharger overboost, we 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. With a digital voltmeter (DVOM), test the 5V supply circuit going to the sensor to ensure it is powered (red wire to the 5V supply circuit, black wire to a good ground). If there are 12 volts at the sensor when there should be 5 volts, repair the wiring from the PCM to the sensor for a short to 12 volts, or possibly a faulty PCM.
If that’s okay, with a DVOM, check that the exhaust pressure sensor signal circuit is 5V (red wire to the sensor signal circuit, black wire to a good ground). If there is no 5 volts at the sensor, or if you see 12 volts at the sensor, repair the wiring between the PCM and the sensor, or again a possible faulty PCM.
If that’s okay, check that the exhaust pressure sensor is properly grounded. Connect a test light to the positive 12V battery terminal (red terminal) 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, that would indicate the circuit problem. If it lights up, wiggle the wire harness going to each terminal to see if the test light flickers, indicating an intermittent connection.
If all tests have passed so far and you continue to get a P0479 code, try wiggling the sensor wire harness while watching the scan tool to see if the code returns. If it does, that most likely indicates an intermittent connection in the harness. Otherwise, it would most likely indicate a faulty exhaust pressure sensor, although a closed exhaust pressure control valve or a faulty PCM cannot be ruled out until the sensor has been replaced.