Technical Description of OBD-II Trouble Code
Range/Performance Issue in Glow Plug Control Module to PCM Communication Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model.
P0684 indicates a problem in the Range/Performance aspect of communication between the Glow Plug Control Module and the PCM. A fault has occurred that prevents the PCM from sending commands to the glow plug control module. The command is essentially a signal to turn the glow plugs on and off.
The codes do not specify a particular part within the system, only the area of failure. The glow plug circuit is relatively simple and can be diagnosed and repaired with minimal automotive knowledge, as long as you have a basic understanding of using a volt/ohmmeter.
What is the purpose of the glow plugs?
To understand their function, you need to grasp the operating principles of a diesel engine.
Unlike a gasoline engine that requires a spark to ignite the fuel, a diesel engine relies on extremely high compression. When air is highly compressed, it generates extreme heat. The diesel engine compresses the air within its cylinders to the point where the air reaches temperatures high enough to spontaneously ignite the fuel.
When a diesel engine block is cold, it is challenging to generate enough heat through compression to ignite the fuel. This is because the cold engine block cools the air, causing a slower increase in temperature that is insufficient for starting.
When the vehicle’s powertrain control module (PCM) detects that the engine is cold based on signals from the oil and transmission temperature sensors, it activates the glow plugs. The glow plugs become red hot and transfer heat to the combustion chamber to assist in starting the engine. They operate on a timer and only function for a few seconds. If they were activated for longer, they would quickly burn out.
How do they operate?
When the PCM senses that the engine is cold, it grounds a glow plug control module (GPCM). Once grounded, the GPCM grounds a glow plug solenoid (similar to a starter solenoid) on the valve cover.
The solenoid then supplies power to a glow plug bus bar. The bus bar has a separate wire for each glow plug. Power travels to the glow plugs, where they heat the cylinder to aid in startup.
The GPCM is a timer and remains activated for only a few seconds. This duration is sufficient to start the engine while also protecting the glow plugs from overheating due to prolonged activation.
Symptoms
Symptoms of a P0684 code may include:
Potential Causes
The potential causes for this DTC may include:
Diagnostic & Repair Steps
Over the years, I have found this to be a common problem with diesel engines regardless of the manufacturer. Due to the high amperage required to operate the glow plugs and their tendency to burn out, I recommend starting with the most common issues.
The GPCM uses low amperage and is least likely to fail, although it is still possible. The solenoid is also rarely replaced. When dealing with high amperage, even a slight looseness in a connection can create an arc and damage the connector.
If the above steps do not identify the problem, consult a service manual and refer to the page depicting the glow plug circuit. Look for the color and pin number for power to the GPCM and power from the GPCM to the solenoid. Use a voltmeter to probe these terminals as directed.
If there is no power to the GPCM, the PCM has failed. If there is voltage to the GPCM but no voltage to the solenoid, replace the GPCM.
