Skip the auto shop - Our mechanics make house calls

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

check engine light

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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...
P2159 OBD-II Trouble Code: Vehicle Speed Sensor B Range/Performance
Diagnostic Trouble Code (DTC): P2159 P2159 code definition Vehicle Speed Sensor B Range/Performance Related Trouble Codes: P2158: Vehicle Speed Sensor B P2160: Vehicle Speed Sensor B Circuit Low P2161:...


Related questions

Q: rough shifting high idle

Hello. This can be caused by a few different things. Most of the time it is caused by the needs for a tune-up or a fuel system problem. Both of these will cause the car to run this way. I...

Q: CHECK ENGINE LIGHT IS ON

There are literally hundreds of possible causes for the check engine light to be illuminated. The check engine light is a generic way to tell you that there is a fault with the vehicle, and that there are diagnostic trouble...

Q: p0420

A vehicle with 182,000 miles is likely due for a catalytic converter. Most of the time it does mean you need a new catalytic converter. But there are a couple of other things that are prudent to take a look...