P0477 Exhaust Pressure Control Valve "A" Low Trouble Code

P0477 Exhaust Pressure Control Valve "A" Low

OBD-II Trouble Code Technical Description

Exhaust Pressure Control Valve "A" Low

What does that mean?

This generic powertrain/engine diagnostic trouble code typically applies to diesel engines, including but not limited to certain Ford, Dodge, Mercedes, Nissan, and VW vehicles.

This code can also apply to those 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 back pressure in the exhaust. This heat and/or back pressure can be used to assist in cold start warm up. It can also be used to oppose cylinder pressure coming from the engine cylinders out of the exhaust, thereby slowing the engine down and the vehicle along with it. This is especially useful during towing operations.

This code is strictly concerned about the incoming signal from the exhaust pressure sensor not matching intake manifold pressure or ambient air pressure during normal driving. This can be a mechanical or an electrical circuit fault, depending upon vehicle manufacturer.

Troubleshooting steps may vary depending upon manufacturer, type of exhaust back pressure control, and wire colors to the control solenoid.

Symptoms

Symptoms of a P0477 engine code may include:

Malfunction Indicator Lamp (MIL) illuminated Lack of power Lack of engine braking Longer than normal cold engine warmup time

Potential Causes

Typically the causes for this code to set are:

Short to Power in the power supply circuit between the exhaust pressure control solenoid and the PCM (powertrain control module) Open in the power supply circuit between the exhaust pressure control solenoid and the PCM Short to ground in the power supply circuit to the exhaust pressure control solenoid Exhaust Pressure Control Relay faulty Exhaust Pressure Control Solenoid faulty Possibly the PCM has failed (highly unlikely)

Diagnostic and Repair Procedures

A good starting point is always a technical service bulletin (TSB) search for your particular vehicle. The vehicle manufacturer may have a PCM flash/reprogram to cover this issue, and it pays to check on this before you find you’ve gone down a long/wrong path. PCM = powertrain control module.

Next, locate Exhaust Pressure Control Solenoid on your particular vehicle. Once located, visually inspect the connectors and wiring. Look for chafing, rubbing, bare wires, burn spots or melted plastic. Pull the connectors apart and carefully inspect the terminals (the metal parts) inside the connectors. See if they look corroded, burnt or possibly green in color versus the normal metal color you are probably used to seeing. You can get some Electrical Contact cleaner at any parts store if cleaning of the terminals is needed. If this is not possible, find some 91% rubbing alcohol and a light plastic bristle brush to clean them with. Afterwards let them air dry, get some dielectric silicone compound (same stuff they use for light bulb sockets and spark plug wires) and put some 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 connections were most likely your problem.

If the code does return, 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 that is in the relay/fuse box that you know works. Clear codes and see if this code returns. If it does not, then the relay was most likely your problem.

If the code does return, we will need to test the solenoid and its associated circuits. Typically there are 2 wires at the Exhaust Pressure Control Solenoid. First, disconnect the harness going to the Exhaus

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