What does it mean?
OBD II fault code P0672 is a generic code that is defined as “Cylinder 2 Glow Plug Circuit/Open”, and is set when the PCM (Powertrain Control Module) detects an electrical defect/failure/malfunction that causes a break in continuity in the control circuitry of the glow plug on cylinder #2. Note though that on most applications, this code will also set when the PCM detects a variation of 10% or more in the required control system voltage.
Note that this code applies only to applications with compression ignition (diesel) engines. 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 older systems, the glow plugs were controlled by a simple relay combined with a timer that controlled the glow plugs’ duty cycle, or “on” time. On these systems, power was fed to the timer/relay when the ignition is turned to the “ON” position, and while these systems worked reasonably well, their biggest drawback was that it could take ten seconds or more to heat the glow plugs sufficiently for the engine to start.
Moreover, these systems’ “ON” time was limited to the amount of time the timer was set to, and if the engine did not start on the first attempt, the cycle had to be repeated. However, 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 advantages of this is that the control module can modulate the current fed to each individual glow plug, which means that all glow plugs are 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 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. 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 both pre-ignition of the air/fuel mixture, and damage to the glow plugs and/or engine.