P0478Generic code

P0478 Code: Exhaust Pressure Control Valve High

The P0478 code indicates high voltage in the control circuit of the exhaust pressure control valve, from an open circuit or a short to voltage. Expect a loss of power, slow warm-up when cold and high exhaust temperature.

Severity
Medium
Can I keep driving?
Yes, but with caution
Estimated cost
$80 to $900
Difficulty
Medium
System
Emissions Control
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0478 code means

On some diesels (Ford Power Stroke, International) a valve in the exhaust partly closes when the engine is cold to raise the engine load and warm it up sooner. It also helps the cabin heater.

The computer monitors the control circuit from its own driver transistor. The P0478 code indicates high voltage in the control circuit (an open circuit or a short to voltage). Before replacing the component, check the fuse, relay, connector and wiring, since they are the most common cause.

If the valve sticks closed, the engine loses a lot of power and runs hot. If it sticks open, the engine is slow to warm up. It often seizes up with soot and rust.

To confirm it, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be less than 1 ohm, and an infinite reading means the wire is open. If the wiring is good, measure the component itself: infinite resistance confirms it is open inside.

P0478 code symptoms

  • Check engine light on.
  • Loss of power.
  • Slow warm-up when cold.
  • High exhaust temperature.

Likely causes of the P0478 code

  1. Connector loose or broken. The most frequent cause, especially after another repair in the area.
  2. Open coil or motor, with infinite resistance in the component.
  3. Control wire cut between the component and the computer.
  4. Short to 12 V, where the control wire touches another wire that carries voltage.

How to diagnose the P0478 code step by step

  1. Command the component with the scan tool and check whether it responds.
  2. Check the fuse and relay.
  3. Measure the supply at the connector: battery voltage with the key on or when it is commanded.
  4. Measure the component resistance and compare it with the manual. Also check the insulation to ground.
  5. Check the control wire for continuity and for shorts.
  6. Command the valve with the scan tool and watch the exhaust pressure.
  7. Inspect the exhaust pressure sensor tube.

How to fix the P0478 code

  • Repair the connector, the wiring or the power supply.
  • Replace the exhaust pressure control valve if it does not respond with correct power and control.
  • Clear the codes and verify that the monitor completes.

Technical reference data

  • Electric or vacuum actuator controlled by PWM.
  • Approximate values only: check the manufacturer data for your engine.

Brand-specific notes

This valve is found on some turbo diesels, and soot is hard on it.

P0478 on Ford

Typical of the Ford Power Stroke 6.0 and 6.4.

P0478 on International

Typical of International engines.

Frequently asked questions about P0478

Can I drive with a P0478 code?
With caution, and keep an eye on engine temperature and power. A P0478 code can leave the valve stuck closed, and that makes the engine run hot.
What does the exhaust pressure control valve do?
On some diesels (Ford Power Stroke, International) a valve in the exhaust partly closes when cold to raise the engine load and warm it up sooner. It also helps the cabin heater.
Do I always have to replace the part?
No. Check the fuses, relays, connectors and wiring first.
How much does it cost to fix P0478?
Roughly $80 to $900, depending on whether it is wiring or the valve itself.
What does the P0478 code mean?
It is a generic OBD2 code that means the control circuit of the exhaust pressure control valve is reading high voltage, from an open circuit or a short to voltage.
What is the most common cause of P0478?
A loose or broken connector, especially after another repair in the area. An open coil or motor and a cut control wire come next.

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