What does it mean?
This generic powertrain diagnostic trouble code (DTC) generally applies to all OBD-II equipped vehicles with an exhaust pressure control valve sensor or switch. This may include, but is not limited to, vehicles from VW, Audi, Toyota, etc. Although generic, the exact repair steps may vary by make/model.
The Exhaust Pressure Control (EPC) valve is a solenoid valve used to regulate backpressure during cold weather. This allows for increased cabin heat, facilitates cold starting, and windshield defrosting.
In most cases, the Powertrain Control Module (PCM) uses information from the Exhaust Back Pressure (EBP) sensor, the Intake Air Temperature (IAT) sensor, and the Manifold Absolute Pressure (MAP) sensor to determine valve control. If the PCM detects a problem with the EPC or IAT, it will disable the ECP. Typically, the ECP is found on diesel engines.
In the codes described in this article, the “B” indicates that the problem concerns a part of the system circuit, rather than a specific symptom or component.
The P04AA code is set when the PCM detects an intermittent problem with the exhaust pressure control valve circuit.
How severe is this DTC?
The severity of this code is moderate to severe. It’s a good idea to address this code as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P04AA trouble code may include:
Check Engine Light illuminated
Increased emissions
Poor engine performance
Hard starting
What are some of the common causes of the code?
Causes
of this code may include:
Faulty exhaust pressure control valve
Wiring problems
Faulty PCM
What are the troubleshooting steps for P04AA?
Start by checking the exhaust pressure control valve and its corresponding wiring. Look for loose connections, damaged wiring, etc. If damage is detected, repair as necessary, clear the code, and see if it returns. Next, check for Technical Service Bulletins (TSBs) related to the issue. If nothing is found, you will need to proceed with step-by-step system diagnostics.
The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to the manufacturer’s diagnostic flowchart.
Check the wiring
Before continuing, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles, and ALLDATA offers single-vehicle subscriptions.
Test the solenoid
Remove the solenoid connector. Use a digital multimeter set to ohms to check the internal resistance of the solenoid. To do this, connect the multimeter between the solenoid B+ terminal and the solenoid ground terminal. Compare the resistance measurement to the factory repair specifications. If the meter displays a reading outside of specifications or Out of Limits (OL) indicating an open circuit, the solenoid must be replaced.
Check the power side of the circuit
Ensure the vehicle has sat for at least a few hours (overnight is preferable) and is cold. Remove the solenoid connector. Ignition on, use a digital multimeter set to DC volts to check for power at the solenoid (typically 12 volts). To do this, connect the multimeter’s negative lead to ground and the multimeter’s positive lead to the solenoid B+ terminal on the harness side of the connector. If voltage is not present, set the meter to ohms (ignition off) between the solenoid connector B+ terminal and the solenoid power supply voltage terminal at the PCM. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity.
If everything is okay up to this point, you will want to check that the PCM is providing power. To do this, turn the ignition on and set the meter to DC volts. Connect the meter’s positive lead to the EPC power supply voltage terminal at the PCM and the negative lead to ground. If there is no reference voltage from the PCM, the PCM is likely faulty. However, PCMs rarely fail, so it’s a good idea to double-check your work up to this point.
Check the ground side of the circuit
Ignition off, use a digital multimeter set to ohms to check for ground continuity. Remove the solenoid connector. Connect the multimeter between the solenoid ground terminal and chassis ground. If the meter reads a numerical value, there is continuity. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the solenoid that will need to be located and repaired.