P0479

Exhaust Pressure Control Valve Intermittent

Indicator

The PCM has detected an abnormal voltage reading (high or low) from the exhaust system back pressure control valve circuit. Exhaust pressure is monitored primarily in turbocharged gasoline and diesel engines.

Code Set Parameters

Exhaust back pressure is controlled by the exhaust back pressure valve. The 5-volt (maximum) variable resistance circuit supplies input voltage to the PCM. The PCM calculates the desired exhaust back pressure using input data from the throttle position sensor, mass air flow sensor, tachometer, etc. Actual exhaust back pressure is then compared with desired exhaust back pressure. The exhaust back pressure valve then regulates back pressure by opening and closing at the PCM’s command. Variations that exceed the manufacturer’s recommended back pressure or circuit resistance will result in a stored trouble code and an illuminated service engine soon lamp.

Symptoms

If this code is stored in the PCM, the exhaust back pressure regulator may be disabled. Some models park the regulator in the open position and some in the closed position. Symptoms may include reduced engine performance due to insufficient turbocharger pressure. Of course, a code will be stored and a service engine soon lamp will be illuminated. Other turbocharger/exhaust related codes may also accompany this code.

Common Causes

In most cases, this code is caused by a clogged back pressure tube and/or exhaust back pressure sensor. However, technicians also report that damaged and burnt electrical wiring and connectors are a possibility. The PCM could also be faulty but this is highly unlikely.

Common Misdiagnosis

This code is frequently confused with an exhaust gas recirculation (EGR) code. Although the exhaust back pressure tube may be located near the EGR valve, this code must be diagnosed in a completely different manner.

Diagnosis

  • The exhaust back pressure system uses a variable capacitance sensor to monitor exhaust back pressure and input data to the PCM
  • The PCM calculates the desired amount of exhaust back pressure by considering inputs from various other engine and drivability sensors
  • Exhaust back pressure is regulated using the exhaust back pressure regulator vent control solenoid
  • The PCM signals this solenoid which, in return opens and closes, allowing exhaust gas back pressure to increase or decrease as required
  • The actual exhaust back pressure sensor voltage (varies between manufacturer’s) is then compared with the manufacturer’s recommended exhaust back pressure sensor voltage
  • If these two readings fail to coincide a code will be stored and the exhaust back pressure system may be disabled. If there are other turbocharger or exhaust codes, diagnose and repair them before attempting to diagnose this code
  • A scanner (or code reader) and a digital volt/ohmmeter will prove to be most helpful when diagnosing this code
  • Several tools may be needed to successfully diagnose this code if no exhaust leaks are detected
  • A suitable scanner (or code reader), a digital volt/ohmmeter, and a manufacturer’s service manual (or the equivalent) will be most helpful in performing a successful diagnosis
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • Proceed by obtaining the back pressure to voltage conversion data and exhaust back pressure system wiring schematic or connector views for your vehicle
  • Place the digital volt/ohmmeter + lead on the sensor reference voltage wire and the - lead on the ground wire
  • Place the digital volt/ohmmeter in such a position as to be able to read it from the driver’s seat
  • Start the vehicle and observe the digital volt/ohmmeter while revving the engine
  • If the exhaust back pressure increases as RPMs increase, the problem may be intermittent
  • If this is the case, clear the code and test drive
  • If the back pressure fails to change with varying RPM levels, suspect a clogged exhaust back pressure tube and/or sensor
  • Remove these two items and clean them thoroughly
  • Reinstall, reset the code, and test drive the vehicle
  • If the exhaust back pressure tube and sensor are not clogged, check for a voltage and ground signal at the exhaust back pressure sensor
  • If either signal is open, use the digital volt/ohmmeter to test the exhaust back pressure sensor and the individual circuits
  • Disconnect the PCM connector prior to checking circuit resistance levels
  • Repair open or shorted wiring, components, or connectors as required. If exhaust back pressure readings fail to change in respect to RPM variations, suspect a problem in the exhaust back pressure regulator vent control solenoid or circuit
  • Use the scanner to activate the exhaust back pressure regulator vent control solenoid while monitoring actual exhaust back pressure as determined by the exhaust back pressure sensor/s
  • If no changes in exhaust back pressure are realized by activating the exhaust back pressure regulator control solenoid, disconnect the electrical connector and use the digital volt/ohmmeter to check for appropriate voltage and ground signals
  • Compare your findings with the manufacturer’s specifications (from the service manual)
  • If voltage and ground signals fail to coincide with manufacturer’s recommendations, disconnect the connector from the PCM (and other related controllers) and perform a resistance and continuity test on all circuits
  • Repair or replace open or shorted wiring, connectors, or components as required
  • Clear the codes and test drive the vehicle to ensure that a successful repair has been done. If an acceptable voltage and ground signal are detected at the exhaust back pressure regulator vent control solenoid connector, perform a resistance test on the solenoid itself
  • Compare your findings with the manufacturer’s specifications and replace the solenoid if it fails to conform
  • If the electrical circuits and the exhaust back pressure vent control solenoid comply with the manufacturer’s specifications, suspect a defective PCM
  • Keep in mind that PCM failure is rare and replacement will require reprogramming