What does it mean?
OBD II fault code P0676 is a generic trouble code that is defined as “Glow plug, cylinder 6 -circuit malfunction”, or in some resources as Cylinder 6 Glow Plug Circuit/Open”. Regardless of the specific wording of the definition though, this code sets when the PCM (Powertrain Control Module) detects an open circuit or other circuit malfunction in the circuits that are associated with the glow plug on cylinder #6.
NOTE: This code only applies to diesel vehicles on which the glow plugs are individually controlled with a dedicated control module. On diesel vehicles that use old-style bus-bar-controlled glow plugs, all the glow plugs are connected in series with a bus bar, and a single power source supplies all the glow plugs with current simultaneously.
The sole purpose of glow plugs on diesel engines is to supply an initial heat source in the cylinder to assist in igniting the diesel/air mixture when the engine is cold, or the engine coolant temperature is below about 100 0 F. Unlike gasoline engines that employ superheated plasma derived from an electrical spark to ignite the fuel mixture, diesel engines use compression pressure to heat the fuel/air mixture to a point that is high enough for the mixture to combust spontaneously.
However, during normal operation when the engine is hot, the air/fuel mixture is partially heated by the cylinders’ internal surfaces during the intake and compression strokes, which contributes to the combustion process. Nonetheless, when the engine is cold, (typically defined as under about 100 0 F) it is very difficult for the compression alone to heat the air/fuel mixture sufficiently to combust.
In some cases, the combustion might initiate, but in cold engines, the combustion process is unstable and difficult to maintain, hence the need for a heat source to “trigger” combustion of the air/fuel mixture in each cylinder until stable combustion can be achieved and maintained in all cylinders.
In terms of operating principles, glow plugs are essentially heaters that employ coils of highly resistive wire embedded in a ceramic powder that disperses the heat over a greater surface area when the coil of wire heats up.
While modern glow plugs still follow this general pattern and work in the same way, their design(s) have been refined to the point where they can heat up to several hundred degrees in about two seconds, as opposed to the ten to fifteen seconds (or sometimes longer) required for older designs to warm up to the same temperature. These refinements were made necessary by the need to establish stable combustion in all the cylinders in as short a time as possible to reduce exhaust emissions.
Therefore, on modern glow plug control systems, a dedicated control module senses the electrical resistance of each glow plug, and based on the electrical resistance of each glow plug, the module uses modulated electrical signals to supply each glow plug with current individually.
The practical advantages of this control mechanism are that- all the glow plugs are heated to the same temperature in the same amount of time, irrespective of differences that might exist between the electrical resistances of two or more glow plugs exhaust emissions are reduced to a minimum because stable combustion is achieved simultaneously in all the cylinders Moreover, modern engine cooling systems also incorporate advanced thermostat designs coupled with one or more dedicated temperature sensors that in combination, prevent coolant circulation until the engine coolant reaches a pre-defined value.
This threshold is typically between 100 0 F and about 104 0 F, at which point the glow plugs will be deactivated when the engine is running, or not be activated when the engine is started to prevent the premature and potentially dangerous detonation of the air/fuel mixture. Thus, when the PCM detects an open circuit in any circuit associated with the affected glow plug, it will recognize that it cannot control the affected glow plug effec