OBD-II Code P0477 — What It Means & How to Fix It

What Does This Error Mean?

P0477 means the exhaust pressure control valve circuit is reading a voltage that is too low. The PCM commanded the valve to operate, but detected a lower-than-expected electrical signal. This usually means there is an open circuit, a bad ground, or a failed valve solenoid. You may notice a loss of engine braking, the exhaust brake not working, or reduced power on some vehicles.

Affected Models

Diesel trucks with exhaust brake or engine brake
Ford Super Duty 6.7L Power Stroke
RAM 2500/3500 with Cummins and exhaust brake option
GM Sierra/Silverado Duramax with exhaust brake
Medium-duty commercial trucks and vans

Common Causes

Open circuit in the wiring to the exhaust pressure control valve solenoid
Corroded or loose connector at the valve leaving the circuit incomplete
Valve solenoid coil has failed internally — burned out winding causes low or no current draw
Blown fuse protecting the exhaust control valve circuit
PCM output driver failure causing insufficient voltage to the valve (uncommon)

How to Fix It

Check the fuse for the exhaust pressure control valve circuit. Look in your owner's manual or fuse box diagram for the correct fuse. Replace any blown fuse and retest.

A blown fuse is the simplest possible cause and takes two minutes to check. Always start here.
Locate the exhaust pressure control valve and unplug its electrical connector. Inspect the connector and pins for corrosion, moisture, or pushed-back terminals.

Reconnect firmly. Wiggle the connector while watching scan tool data. If readings jump, the connector is the fault.
Use a multimeter to measure resistance across the solenoid terminals on the valve itself. Refer to your service data for the correct specification — most solenoids measure between 5 and 30 ohms. An open reading (infinite resistance) means the solenoid coil is burned out.

This test confirms whether the valve itself has failed electrically.
Check for voltage at the connector with the ignition on. If you have no voltage on the power supply wire, trace the circuit back to the fuse box and PCM to find the break.

A wiring diagram for your specific vehicle makes this much faster. Most auto parts stores can print one for you.
Replace the valve solenoid or the entire exhaust control valve assembly as needed. Clear the code and confirm the exhaust brake operates correctly on a test drive.

On some vehicles the solenoid is replaceable separately. On others the whole valve assembly must be replaced.

When to Call a Professional

If basic wiring checks do not resolve this code, a diesel shop with a wiring diagram and oscilloscope can find the fault quickly. Solenoid replacement on most valves is straightforward. Expect $100 to $300 total for diagnosis and repair.

Frequently Asked Questions

What is the exhaust brake and why does it matter?

An exhaust brake is a system that restricts exhaust flow to slow the engine without using wheel brakes. It is especially valuable when towing heavy loads on long downhill grades. When P0477 disables the exhaust brake, you rely entirely on your service brakes. This increases wear and can cause brake fade on steep descents with heavy loads.

Can a bad ground cause P0477?

Yes. If the ground wire for the valve is corroded or loose, current cannot flow through the solenoid properly. This creates a low-current condition that the PCM interprets as a low signal fault. Check ground connections on the valve and in the nearby engine wiring harness.

Is P0477 a serious code?

For light-duty driving, it is moderate — you will notice the exhaust brake is inactive. For heavy towing or trucks that rely on engine braking, it is more serious. In commercial trucking, a non-functional exhaust brake can be a safety and compliance concern. Address it promptly if you tow heavy loads regularly.