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P0477 OBD-II Trouble Code: Exhaust Pressure Control Valve Low

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P0477 code definition

The P0477 code means that the PCM has detected an abnormally low voltage reading from the exhaust back pressure control valve circuit. This code may be stored along with other codes that relate to the exhaust back pressure control system, such as the P0478 and P0479 codes.

What the P0477 code means

The exhaust back pressure valve controls the exhaust back pressure. The circuit connected to the valve obtains data regarding the pressure in volts, and sends this data to the PCM. If the PCM registers that the actual voltage of the exhaust back pressure is lower than the manufacturer’s specifications, it will store the P0477 code.

What causes the P0477 code?

In many cases, this code is caused by a clogged back pressure tube, or a clogged exhaust back pressure sensor. There may also be damaged or burnt electrical wiring that has caused this code to be stored.

Other causes may include:

  • Faulty exhaust back pressure valve.
  • Opened or shorted exhaust back pressure valve harness.
  • Poor electrical connection in the exhaust back pressure valve circuit.
  • Faults in the exhaust back pressure (EBP), the engine oil temperature (EOT), or the intake air temperature (IAT) sensors.

In some very rare cases, a faulty PCM may be to blame for the P0477 code being stored.

What are the symptoms of the P0477 code?

Before the P0477 code is detected, there may be some detectable symptoms. These include reduced engine performance, insufficient turbocharger pressure, and the illumination of the Check Engine Light.

How does a mechanic diagnose the P0477 code?

In order to diagnose the P0477 code, an OBD-II scanner is necessary. After obtaining all stored codes from the PCM, the mechanic can begin by visually inspecting all the electrical wiring, connectors, and other components related to the exhaust back pressure system. If these are repaired, replaced, or already in good condition, then the mechanic may move on to other repair options.

Common mistakes when diagnosing the P0477 code

As with other codes related to the exhaust back pressure system, it is common to misdiagnose this code as having something to do with the exhaust gas recirculation system. These two systems are located near each other, but are diagnosed and repaired in totally different ways.

It is also common to forget to clear the PCM memory of all codes and perform a retest of the system after each potential repair. This step is important, because it helps narrow down the possible root cause until the malfunction is found and repaired.

How serious is the P0477 code?

The P0477 code is a moderately serious code. Left un-repaired, the issue that caused the P0477 code may have an effect on the performance of the vehicle, causing it to perform at a less than optimal level. Additionally, it is likely that the vehicle would not pass an emissions test while the issue that caused the code remains un-repaired.

What repairs can fix the P0477 code?

Repairing the issue that caused the PCM to store the P0477 code can take many forms. Repairs may include:

  • Unclogging or repairing the back pressure tube.
  • Cleaning, repairing, or replacing the exhaust back pressure sensor.
  • Repairing or replacing a faulty exhaust back pressure valve.
  • Repairing or replacing an opened or shorted exhaust back pressure valve harness.
  • Repairing a poor electrical connection in the exhaust back pressure valve circuit.
  • Replacing faulty exhaust back pressure valve solenoids.
  • Repairing or replacing any faulty electrical components, such as wiring or connectors.
  • Replacing a faulty PCM in certain rare cases.
  • Diagnosing and repairing any issues related to other codes stored in the PCM regarding the back exhaust pressure system.
  • Repairing or replacing faulty EBP, EOT, or IAT sensors.

Need help with a P0477 code?

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