Error Code P0478 is defined as Exhaust Pressure Control Valve “A” High, meaning there’s an abnormally high voltage reading detected, due to a problem in the exhaust system back pressure control valve circuit.

This code is a generic trouble code and typically applies to diesel engines, including but not limited to certain models of Ford, Dodge, Mercedes, Nissan, and VW vehicles. It can also apply to trucks with diesel engines and dealer installed exhaust brakes.


To generate heat in the form of back pressure in the exhaust, a valve is placed in the exhaust stream, after the exhaust manifold. This is essential for cold start warm up, and to oppose cylinder pressure coming from the engine cylinders out of the exhaust, thereby, slowing the engine down and the vehicle along with it. This can be extremely useful during towing operations.

The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) calculates the right exhaust back pressure based on the data sent from various sensors, as defined by the manufacturer’s specifications. Then it compares the value with the actual exhaust back pressure reading, which is controlled by the back pressure valve. If the value doesn’t correspond to each other (too high), then the PCM will register the Error Code P0478.

Common Symptoms

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

  • Lack of engine power
  • Lack of engine braking
  • Engine warm-up time taking longer than normal

Possible Causes

In most cases, the cause of this code is blocked or clogged back pressure tube, or clogged exhaust back pressure sensor. Burned and damaged electrical wiring can also cause this code.

Other possible causes include:

  • Defective exhaust back pressure valve
  • Opened or shorted exhaust back pressure valve harness
  • Poor electrical connection in exhaust back pressure valve circuit
  • Faults in EBP (exhaust back pressure), EOT (engine oil temperature), IAT (intake air temperature) sensor
  • Exhaust Pressure Control Solenoid faulty
  • Exhaust Pressure Control Relay faulty
  • Short to ground in the power supply circuit to the exhaust pressure control solenoid
  • Faulty PCM (rare)

How to Check

One of the most basic ways to diagnose this code is by performing an exhaust back pressure step test. This test forces the PCM to measure the pressure values, and then compare them to the rate at which the pressure decays while the vehicle is running. The results of this test will show there is a problem with the exhaust back pressure system.

How to Fix

Common repairs for this code include:

  • Verification of the code using the OBD_II scanner, or through an exhaust back pressure step test.
  • Visual inspection of the electrical components in the exhaust back system and repair of corroded, loose, defective, open, or shorted connections.
  • After each possible repair, make sure to clear the codes and then perform a road test to ensure the repair has properly addressed the cause.
  • Next, perform a resistance test on the solenoid and repair or replace the defective solenoid.
  • Repair or replace defective exhaust back pressure valve harness.
  • If all repairs have been attempted and completed, and the Error Code P0478 still prevails, then the problem may be caused by a defective PCM. This requires replacement of the PCM.

Because voltage values are compared to values calculated from information generated from different sensors, such as throttle position sensor, mass airflow sensor, or tachometer, then it may be necessary to check these sensors to ensure they are working properly. One way to pinpoint a defective sensor is to perform a manual check of the voltage in the exhaust back pressure system and comparing your results to the specifications set by the manufacturer. If the values add up, then there must be a faulty sensor elsewhere.

Error Code P0478 is a moderately serious problem. If left unchecked, this code can affect the performance of the vehicle, causing it to perform at a less optimal level. Also, there’s a good chance the vehicle will not pass emission test while the code is unrepaired. However, the vehicle may remain operational before this problem is fixed.