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Error Code P0479 is defined as Exhaust Pressure Control Valve “A” Intermittent. This means the exhaust pressure control valve circuit is having an out-of-ordinary voltage reading, which could be caused by damage to the wiring.
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.
For diesel or turbocharged gasoline engine, the exhaust back pressure valve is responsible for controlling exhaust pressure. The PCM automatically (powertrain control module, also known as ECM or engine control module in other vehicle makes) determines the required exhaust back pressure based on the manufacturer’s specifications, using data sent by the Throttle Position sensor, Tachometer, and other sensors.
When there is an out-of-ordinary voltage reading (higher or lower than the desired pressure) seen from the exhaust pressure control valve circuit, the Error Code P0479 will be set.
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
Common possible causes for this code include:
- Clogged exhaust
- Stuck exhaust back pressure valve
- Signal circuit between PCM and exhaust pressure sensor open
- Circuit to the exhaust pressure sensor open in the ground
- Signal circuit to the exhaust pressure sensor short to voltage
- Defective exhaust pressure sensor – internal short to voltage
- Possible turbocharge overboost
- Failed PCM (rare)
How to Check
As with most error codes, mechanics diagnose this code using an OBD-II scanner. They start their diagnosis with a thorough visual inspection and check for signs of damage to the connectors, wirings, and other components of the exhaust pressure control system.
After fixing these problems and clearing the code, technicians will then take the vehicle for a test drive to see if the code comes back or if the Check Engine light lights up again. If it does, then technicians proceed on pursuing other repair options, such as checking the Exhaust Pressure Sensor and dislodging carbon buildup inside; repair or replacement of broken, burnt, or chaffed wiring, corroded or burnt connector terminals; connection problem, etc.
Diagnosing this code requires a lot of knowledge of the various involved components and advanced tools. Thus, it is highly recommended to have a certified and well-equipped professional to take care of this problem.
How to Fix
Common repairs for this code include:
- Repair or replacement of damaged wires and connectors
- Removing and cleaning of the exhaust back pressure tube and sensor
- Repair or replacement of shorted connectors or wirings of all circuits
- Replacement of solenoid
This code directly relates to the performance of the engine, which means it requires a serious and thorough diagnosis and repair as soon as possible. Keeping this code unchecked will result to increase in fuel consumption. The vehicle will also be likely to fail an emission test with this code present in the PCM.