Error Code P04AA is defined as Exhaust Pressure Control Valve B Circuit Intermittent. This is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system with an exhaust pressure control valve sensor or switch. This includes vehicle models from Audi, Toyota, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

The EPC or Exhaust Pressure Control is a solenoid controlled valve that is included to regulate the back pressure during cold temperature. This is typically found in diesel engines. This is important during cold starting, as it helps in increasing cabin heat and defrosting windshield. The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) uses the information from the EBP (exhaust back pressure), IAT (intake air temperature sensor), and MAP (manifold absolute pressure) sensor. If the PCM sees a problem with the EPC or the IAT, it will disable the EPC.

Error Code P04AA is set when the PCM detects an intermittent problem with the EPC valve circuit. The “B” on Exhaust Pressure Control Valve B Position Sensor refers to the part of the system circuit, and NOT on a particular component or symptom.

Common Symptoms

As with other codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other common symptoms include:

  • Had starting
  • Increase emissions
  • Poor engine performance

Possible Causes

Common causes for this code include:

  • Defective wiring
  • Defective EPC valve
  • Defective PCM

How to Check

Start the diagnosis by checking the EPC control valve and its wirings. Look for signs of damage and loose connections. Any damages found must be repaired, if not replaced. Clear the codes and see if the code returns.

Make sure you check with the TSB (technical service bulletins) regarding this code. Many times manufacturers have provided specific thorough procedures to fix this code.

For the wiring:

Check with your factory wiring diagram to determine which wires are which. You can find a free repair manual online.

For the solenoid:

Remove the solenoid connector, then set the digital multi-meter at ohms and check the solenoid for internal resistance. You can do this by connecting the meter between the solenoid B+ terminal and the solenoid ground terminal. Compare your results to the specifications in the repair manual. If the meter shows a reading that is out of the specifications, or out of limits (OL) indicating an open circuit, the solenoid should be replaced.

For the power side of the circuit:

When the vehicle is still cold (make sure the vehicle has sat for at least a few hours or overnight), take out the solenoid connector. With vehicle ignition on, set the digital multimeter to DC and check for power at the solenoid (this should be 12V). You can do this by connecting the negative meter lead to ground and the positive to the solenoid B+ terminal on the harness side of the connector. If there is no voltage, set the meter to ohms (with the ignition off) between the B+ terminal on the solenoid connector and the solenoid supply voltage terminal on the PCM. If the meter shows OL (out of limits), then that means there’s an open circuit between the PCM and the sensor, which means it must be located and repaired. If you see a numeric value on the meter, then there’s continuity.

If everything looks fine, check the power that comes out of the PCM. Turn the ignition on and then set the meter to DC volts. Connect the positive meter lead to the EPC supply voltage terminal on the PCM and the negative lead to the ground. If you find no reference voltage from the PCM, then that probably means you have a defective PCM.

Note that, PCM rarely go bad, so make sure you double check at this point.

For the ground portion of the circuit:

With the vehicle ignition off, check for continuity to ground using a digital multimeter set to ohms. Remove the solenoid connector. Connect the meter between the solenoid ground terminal and chassis ground. If the meter reads a numeric value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and solenoid that will need to be located and repaired.

How to Fix

Common repairs for this code are:

  • Repair or replacement of solenoid
  • Repair the open circuit between the PCM and the sensor
  • Replacement of PCM

The is a generic way to check and fix this code. To accurately test the system, you will need your manufacturer’s diagnostic flow chart.