Technical Description of OBD-II Trouble Code
The exhaust pressure control valve “A” is operating at a low level.
What does this mean?
This generic diagnostic trouble code is typically found in diesel engines, specifically in certain Ford, Dodge, Mercedes, Nissan, and VW vehicles. It can also apply to trucks with diesel engines and dealer-installed exhaust brakes. The purpose of the valve is to generate heat and back pressure in the exhaust, which can assist in cold start warm-up and slow down the engine 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.
The troubleshooting steps may vary depending on the manufacturer, type of exhaust back pressure control, and wire colors to the control solenoid.
Symptoms
The symptoms of a P0477 engine code may include:
Potential Causes
The typical causes for this code to set are:
Diagnostic and Repair Procedures
A good starting point is always to search for a technical service bulletin (TSB) for your particular 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 Control Solenoid on your specific vehicle. Once found, visually inspect the connectors and wiring for any signs of damage such as chafing, rubbing, bare wires, burn spots, or melted plastic. Disconnect the connectors and carefully inspect the terminals inside for corrosion, burning, or unusual discoloration. If cleaning is necessary, use Electrical Contact cleaner or rubbing alcohol and a light plastic bristle brush. After cleaning, let them air dry and apply dielectric silicone compound to the terminals.
If you have a scan tool, clear the diagnostic trouble codes from memory and see if this code returns. If it doesn’t, then the issue was likely with the connections.
If the code does return, locate the relay that operates the Exhaust Pressure Control solenoid. This relay may also be referred to as an Exhaust Brake or Engine Brake relay. Swap it with an identical relay that is known to work. Clear the codes and see if this code returns. If it doesn’t, then the relay was likely the problem.
If the code does return, further testing of the solenoid and its circuits is necessary. Typically, there are 2 wires connected to the Exhaust Pressure Control Solenoid. Using a Digital Volt Ohm Meter (DVOM), check the resistance between the solenoid terminals. It should not be open or shorted. Refer to the resistance specifications for your specific vehicle. If the solenoid is open or shorted, replace it.
If the solenoid is okay, use a DVOM to check for 12V on the power supply circuit of the Exhaust Pressure Control Solenoid. Connect the red lead of the meter to the solenoid power supply circuit and the black lead to a good ground. Make sure the ignition is on. If there is no 12 volts or if there is 12 volts when the ignition is turned off, repair the wiring from the PCM or relay to the solenoid, or consider a possible PCM failure.
If the power supply circuit is okay, check for a good ground at the Exhaust Pressure Control Solenoid. Connect a test light to the positive terminal of a 12V battery and touch the other end to the ground circuit of the solenoid. If the test light doesn’t illuminate, there is a problem with the circuit. If it does illuminate, wiggle the wiring harness going to each sensor to check for intermittent connections.
If all tests have passed and the P0477 code continues to appear, it is likely that the Exhaust Pressure Control Solenoid has failed. However, a failed PCM cannot be ruled out until the solenoid has been replaced.
