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Error Code P0477 is defined as Exhaust Pressure Control Valve “A” Low. This means there’s an abnormally low voltage reading, usually caused by the exhaust 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 for towing operations.
Error Code P0477 means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), has detected an abnormally low voltage reading is coming off of the exhaust back pressure control valve circuit. This code may be stored together with codes that relate to the exhaust back pressure control system, such as:
- Error Code P0478
- Error Code P0479
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
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
As with other error codes, an OBD-II scanner is necessary to diagnose this code.
After getting all the stored codes from the PCM, technicians start their diagnosis with a visual inspection of all connectors, wirings, and other components related to the exhaust backpressure system.
If faulty components are fixed (repaired and replaced) and are in good condition, then the mechanic can proceed to next repair options.
How to Fix
Common fixes for this code include:
- Unclogging or repair of back pressure tube
- Cleaning, repair, or replacement of the exhaust back pressure sensor
- Repair or replacement of defective exhaust back pressure valve
- Repair or replacement of opened or shorted exhaust back pressure valve harness
- Replacement of defective exhaust back pressure valve solenoids
- Repair or replacement of defective electrical components (connectors or wirings)
- Replacement of defective PCM (in rare cases)
- Diagnosing and repair of other related codes stored in the PCM regarding the back exhaust pressure system
- Repair or replacement of defective EBP, EOT, or IAT sensors
Error Code P0477 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 an emissions test while the code is unrepaired.