P0478 OBD-II Trouble Code: Exhaust Pressure Control Valve High

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P0478 is a diagnostic trouble code (DTC) for "Exhaust Pressure Control Valve High". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0478 code definition

In the event that the PCM stores the P0478 code, it means that there has been an abnormally high voltage reading from the exhaust system back pressure control valve circuit. This code is frequently associated with the P0479 code, which registers an intermittently abnormal voltage reading from the same system.

What the P0478 code means

The PCM calculates the proper exhaust back pressure based on data from various sensors, according to the manufacturer’s specifications. It then compares this value with the actual exhaust back pressure reading, which is controlled by the exhaust back pressure valve. If the actual voltage value is too high, the PCM will store the P0478 code.

What causes the P0478 code?

Usually, clogged back pressure tubes or faulty back pressure sensors are to blame when the P0478 code is stored.

Other causes may include:

  • Damaged, corroded, loose, or shorted electrical components, such as wiring or connectors
  • Open or shorted exhaust back pressure valve harness
  • Poor connection in the exhaust back pressure valve circuit
  • Faulty PCM in some rare cases

What are the symptoms of the P0478 code?

Because this code may be stored due to a back pressure regulator being disabled, one of the most common symptoms is insufficient turbocharger pressure, resulting in reduced engine performance. The Check Engine Light will be illuminated, and other codes related to the exhaust back pressure system, or the turbocharger system, may also be stored in the PCM.

How does a mechanic diagnose the P0478 code?

One of the ways that the P0478 code may be diagnosed is by performing an exhaust back pressure step test. This test forces the ECM to measure pressure values, and compare them to the rate at which the pressure decays while the vehicle is running. The results of this test will likely show that there is a fault within the exhaust back pressure system.

Common mistakes when diagnosing the P0478 code

Because the exhaust back pressure tube is often located very near to the exhaust gas recirculation valve, it is common to misdiagnose the issue of the P0478 code as one relating to this EGR system. These two systems must be diagnosed and repaired in different manners.

How serious is the P0478 code?

If the P0478 code is stored in the PCM, it is considered a moderately serious matter. It will probably affect vehicle performance and fuel economy, and may also cause the vehicle to fail an emissions test. However, the vehicle may remain operational before this issue is repaired.

What repairs can fix the P0478 code?

The repairs that can be performed when the P0478 code is detected include:

  • Verify the code with an OBD-II scanner or an exhaust back pressure step test.
  • Perform a visual inspection of the the electrical components in the exhaust back system, and repair any that are loose, corroded, faulty, open, or shorted.
  • After each possible repair has been performed, be sure to clear the code and perform a road test, to ensure that the repair addressed the proper cause.
  • Perform a resistance test on the solenoid, and repair or replace a faulty solenoid.
  • Replace or repair a faulty exhaust back pressure valve harness.
  • If all these repairs have been attempted, and the P0478 code continues to be stored, it may be necessary to replace the PCM.

Because the voltage values are compared to values calculated from information that is gathered from a variety of sensors, such as the throttle position sensor, tachometer, or mass air flow sensor, it may be necessary to check these sensors and ensure that they are working correctly. One way to pinpoint a faulty sensor is to perform a manual check of the voltage in the exhaust back pressure system, and manually compare with the manufacturer’s specifications. If the values line up, then suspect a fault sensor elsewhere.

Need help with a P0478 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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