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 may also be used to oppose cylinder pressure from the engine 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 P0477 engine code may include:
Malfunction Indicator Lamp (MIL) illuminated
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 as follows:
Short to power in the supply circuit between the exhaust pressure control solenoid and the PCM (Powertrain Control Module)
Open in the supply circuit between the exhaust pressure control solenoid and the PCM
Short to ground in the exhaust pressure control solenoid supply circuit
Faulty exhaust pressure control relay
Faulty exhaust pressure control solenoid
Possibly a failed PCM (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/reprogramming to address 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 control solenoid on your specific vehicle. Once located, 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 appear corroded, burnt, or perhaps greenish compared to the normal metal color you’re 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 (the same 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, locate the relay that operates the exhaust pressure control solenoid. This relay may also be called an exhaust brake or engine brake relay. Once located, swap it with an identical relay in the relay/fuse box that you know is working. Clear the codes and see if this code returns. If not, the relay was likely your problem.
If the code returns, we’ll need to test the solenoid and its associated circuits. Typically, there are 2 wires to the exhaust pressure control solenoid. First, disconnect the wiring harness from the exhaust pressure control solenoid. With a digital volt-ohm meter (DVOM), connect one meter lead to one terminal of the solenoid. Connect the remaining meter lead to the other terminal of the solenoid. It should not be open or shorted. Check the resistance specifications for your specific vehicle. If the solenoid is open or shorted (infinite resistance or no resistance/0 ohms), replace the solenoid.
If that’s okay, with a DVOM, check that the exhaust pressure control solenoid supply circuit has 12V (red lead to the solenoid supply circuit, black lead to a good ground). Make sure the ignition is on. If there is no 12 volts at the solenoid, or if there is 12 volts when the ignition is off, repair the wiring from the PCM or relay to the solenoid, or possibly a faulty PCM.
If that’s okay, check that the exhaust pressure control solenoid 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 control solenoid circuit ground. If the test light does not illuminate, that would indicate a circuit problem. If it lights up, wiggle the wiring harness going in each direction