Technical Description of OBD-II Trouble Code
The circuit of the Glow Plug Control Module is experiencing low voltage.
What does that mean?
The OBD code P0383 is a generic code that applies to all brands of modern diesel engines, including Ford, Dodge, Chevrolet, GMC, and VW Volkswagen vehicles. To understand the significance of this code, its effects, and symptoms, it is important to understand how diesel engines work.
Unlike gasoline engines, diesel engines do not rely on a compressed fuel mixture and an electrical ignition source. Diesel engines have much higher compression than gasoline engines.
This high compression causes the air in the cylinder to reach temperatures over 600 degrees, which is enough to ignite the diesel fuel. When the piston reaches the top dead center of the cylinder, high-pressure fuel is sprayed into the cylinder. It immediately ignites when it comes into contact with the super-heated air, and the expanding gases drive the piston down.
Glow Plug
Since a diesel engine requires super-heated air to ignite the fuel, a problem arises when the engine is cold. When starting a cold engine, it is difficult to super-heat the air because its heat is quickly lost to the cold cylinder head.
The glow plug provides the solution. Installed in the cylinder head, the pencil-shaped plug heats up until it glows for up to ten seconds. This raises the temperature of the surrounding cylinder wall, allowing the compression heat to rise sufficiently for ignition.
A typical diesel engine glow plug:
Glow Plug Circuit
The circuit is common to all diesel engines, except for the component used to time the glow plug operation. The vehicle will either have a glow plug control module or the PCM will handle this. If you don’t have a service manual, you can call an auto parts store and ask if they sell a control module. If not, then the computer controls the timing.
Operating Principles
When the ignition is on, it supplies power to the glow plug relay. The computer or control module will supply a ground to activate the relay. The oil temperature sensor is the determining factor. When the computer detects a cold engine, it will either activate the control module or relay by supplying the ground.
When activated, the relay supplies power to the glow plugs for a duration determined by the computer or control module.
If the vehicle has a control module, all it does is supply a timed ground to the relay. It will have a fused power source, and the computer supplies the ground to turn it on.
A P0383 code is set when the PCM (powertrain control module) detects a low circuit condition in the glow plug control module electrical circuit.
Symptoms
The glow plug warning light will illuminate, and the engine will be slow to start in warm weather or fail to start in cold weather.
If the engine does start, there will be a noticeable knocking noise until the engine warms up to operating temperature. White smoke will be visible from the exhaust as the excess fuel from the hard start burns off. The engine will have a noticeable misfire until the cylinder head temperatures rise sufficiently to support complete combustion.
Glow plug warning lamp illuminated:
Potential Causes
The glow plugs have reached the end of their useful life and need replacement. Poor injection timing can cause excessive wear on the glow plug. Additionally, a stuck glow plug relay or timer module can cause them to burn out faster.
Diagnostic Steps & Possible Solutions
Other Glow Plug related DTCs: P0380, P0381, P0382, P0383, P0384, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P0683, P0684.
