What does it mean?
OBD II fault code P0684 is a generic code that is defined as “Glow Plug Control Module 2 to PCM Comm. Circuit Range/Performance”, and is set when the PCM (Powertrain Control Module) detects an abnormal, out-of range, or implausible voltage in the communications circuits between itself and the glow plug control module. NOTE: Note that on a small number of applications, such as some V-type engines, there are two glow plug control modules- one for each bank of cylinders.
In the case of code P0684, the code refers to the glow plug control module that controls the glow plugs on Bank 2, which is the bank of cylinders that does not contain cylinder #1. Although diesel engines rely on compression alone to ignite the air/fuel mixture, ignition of the mixture is very difficult to accomplish when the engine is cold.
Therefore, all diesel engines are fitted with glow plugs whose tips extend into the pre-combustion chamber, and whose purpose it is to assist in the initial heating of the air/fuel mixture when the engine is cold. In terms of operation, a glow plug is a metal cylinder that contains a heating element that is in its turn, embedded in a ceramic material that transports the elements’ heat to the outside metal wall of the glow plug through convection.
When a current is applied to the glow plug, the element heats the plug’s tip to a very high temperature, which serves as a heat source to initiate ignition of the air/fuel mixture in cold engines. On modern systems, the glow plugs are controlled by a dedicated glow plug control module that has the ability to sense the electrical resistance of each individual glow plug.
The practical advantage of this is that the control module can modulate the current that is fed to each individual glow plug, which means that all the glow plugs can be heated to the same temperature in the same amount of time. Consider the image below- This image shows the progression of heat through a single glow plug from cold, to fully heated.
While this process on older systems could take ten seconds or more, improvements in glow plug design coupled with improved electronic control systems means that all the glow plugs on an engine can now be heated in less than two seconds. This not only reduces start-up times even in sub-zero temperatures, but also extends the lives of the glow plugs and other components such as the starter motor.
As a practical matter, modern glow plug control systems are relatively complex, and although the glow plug control module controls and monitors the actual heating of the glow plugs, the PCM needs to “know” exactly what is happening in the glow plug control circuit at all times to manage the overall fuel delivery and other engine management strategies effectively, and particularly during start up and the few seconds following start up.
Therefore, the PCM monitors the glow plug control module and its associated circuits very closely via dedicated communication circuits, and if any fault, defect, malfunction, or failure occurs that prevents effective communication between the PCM and the glow plug control module, the PCM will set code P0684, and illuminate a warning light. Nonetheless, it must be noted that glow plugs are not required to start a hot or even warm engine.
To prevent the glow plugs on both old and newer systems from being activated when the engine is hot, the PCM uses input data from a dedicated engine coolant temperature sensor. When the coolant temperature exceeds a predefined limit, which is usually about 104 0 F, the PCM deactivates the glow plug control system to prevent premature ignition of the air/fuel mixture, which could cause damage to the glow plugs and/or the engine.