High “B” Exhaust Pressure Control Valve
What does it mean?
This generic powertrain/engine diagnostic code typically applies to diesel engines, including but not limited to certain Ford Powerstroke, Dodge Cummins, 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 valve is the “B” valve in your particular case.
Symptoms
Symptoms of a P04A2 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 of P04A2
Typically, the causes for this code to set are:
Stuck exhaust backpressure valve
Restricted exhaust
Open in the ground circuit to the exhaust pressure sensor
Open in the signal circuit between the exhaust pressure sensor and the PCM
Short to voltage in the exhaust pressure sensor signal circuit
Faulty exhaust pressure sensor – internal short to voltage
Clogged exhaust pressure sensor sensing tube
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 address this issue, and it’s worth checking before discovering you’ve taken a long/wrong path. PCM = Powertrain Control Module.
Next, locate the “B” exhaust pressure sensor 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 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 available, find 91% isopropyl alcohol and a soft plastic-bristled brush to clean them. Then let them air dry, get some dielectric silicone compound (the same kind used for bulb sockets and spark plug wires) and apply a small amount where the terminals make contact.
Additionally, if your vehicle is equipped, remove the sensing tube that connects the exhaust backpressure sensor to the exhaust manifold. Try blowing through it. If it’s blocked, this is also a possible cause of this code.
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, you’ll need to check the proper operation of the turbocharger overboost. You will need a scan tool capable of reading the turbocharger overboost pressure. You may need to monitor the intake manifold pressure, as this will provide the same information. Note the pressure with the Key in the On position, 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 RPM 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 was successful, 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 Volt-Ohm Meter (DVOM), test the 5V supply circuit going to the sensor to ensure it is powered (red lead to the 5V supply circuit, black lead 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 at 5V (red lead to the sensor signal circuit, black lead 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 from the PCM to the sensor, or again a faulty PCM.
If that checks out, verify 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, this would indicate a circuit problem. If it lights up, wiggle the wire harness going to the exhaust pressure sensor to see if the test light flickers, indicating an intermittent connection.
If all the previous tests were successful and you continue to get a P04A2 code, this would most likely indicate a failing exhaust pressure sensor, although a stuck closed exhaust backpressure valve or a failing PCM cannot be ruled out until the sensor has been replaced.