P0477

What it means

P0477 means the engine control unit has detected that the exhaust pressure control valve 'A' circuit is reading low. In practice the ECU commands the valve and sees a signal voltage or feedback that is lower than expected, which usually points to an electrical fault in the circuit or a fault in the valve itself. The valve is typically found on diesel applications with exhaust back-pressure control, but fitment and layout depend on the model and engine.

Symptoms

• Engine warning light on
• Reduced power or the car going into limp mode
• Poor or uneven warm-up, especially in cold weather
• Increased exhaust smoke or a noticeable change in exhaust note
• Higher fuel consumption
• Rough running or hesitation under load

Possible causes

• Wiring fault in the valve circuit, such as a short to earth, chafed insulation or an open circuitlikely
• Corroded, loose or damaged connector at the valve or at the control unitlikely
• Exhaust pressure control valve seized, sticking or failed internally, often from soot build-uplikely
• Faulty valve position feedback sensor or actuator motorpossible
• Blocked or leaking vacuum or air supply to the valve, where the system is pneumatically operatedpossible
• Low supply voltage or a poor earth affecting the circuitpossible
• Fault within the engine control unit driver circuitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including engine speed, load, temperature and battery voltage
2. Check for other related codes, such as exhaust pressure sensor, turbocharger or particulate filter faults, and deal with those in sensible order
3. Visually inspect the valve, its wiring harness and connectors for heat damage, chafing, corrosion, moisture or loose pins
4. Check battery condition and charging voltage, as well as the relevant fuses, supply and earth connections
5. Using a scan tool with live data, command the valve if the function is available and watch whether it moves and whether the reported position follows the command
6. Measure the supply, earth and signal voltages at the valve connector against the manufacturer's specification for that model and engine
7. Check wiring continuity and insulation resistance between the valve and control unit, looking for shorts to earth
8. Inspect the valve for sticking, soot build-up or mechanical damage, and check any vacuum lines and supply to it where fitted

How to fix

• Repair or replace damaged wiring and renew faulty connector terminals
• Clean or replace corroded or contaminated connectors and secure them properly
• Free off and clean the valve if the manufacturer permits it, otherwise replace the exhaust pressure control valve
• Replace the actuator or position sensor if it has failed and is available separately
• Repair vacuum or air supply leaks and replace any faulty control components in that supply
• Rectify charging or earthing problems, then clear the code and road test to confirm it does not return

Can I keep driving?

Nearby codes

• P0476GenericExhaust Pressure Control Valve A Range/Performance
• P0478GenericExhaust Pressure Control Valve A High