What Does It Mean?
This generic powertrain/diagnostic engine 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 depending on the manufacturer, the type of exhaust backpressure control, and the color of the control solenoid wires. Refer to a vehicle-specific repair manual to determine which is the “B” valve in your particular case.
Symptoms
Symptoms of a P04A1 engine code may include:
Malfunction Indicator Lamp (MIL) illuminated
Lack of power
Lack of engine braking
Cold engine warm-up time longer than normal
Potential Causes of P04A1
Typically, the causes for this code to set are:
Short to power in the power circuit between the exhaust pressure control solenoid and the PCM (Powertrain Control Module)
Open in the power circuit between the exhaust pressure control solenoid and the PCM
Short to ground in the exhaust pressure control solenoid power 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/reprogram to cover this issue, and it’s worth checking before finding out you’ve taken a long/wrong path. PCM = Powertrain Control Module.
Next, locate the “B” 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. Disconnect the connectors and carefully inspect the terminals (the metal parts) inside the connectors. See if they look corroded, burnt, or perhaps green in color 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 a bit where the terminals make contact.
If you have a scan tool, clear the diagnostic codes from memory and see if this code returns. If it doesn’t, 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 it doesn’t, the relay was likely your problem.
If the code returns, we will 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 power circuit is 12V (red lead to the solenoid power circuit, black lead to a good ground). Ensure 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 ground is good. 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 the circuit problem. If it lights, wiggle the wiring harness going to each sensor to see if the test light flickers, indicating an intermittent connection.
If all tests have passed so far and you continue to get a P04A1 code, it would most likely indicate a failing exhaust pressure control solenoid, although a faulty PCM cannot be ruled out until the solenoid has been replaced.