|Trouble Code||Fault Location||Probable Cause|
|P0676||Glow plug, cylinder 6 -circuit malfunction||Wiring, poor connection, relay, glow plug control module, glow plug, ECM|
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What Does Code P0676 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 1000F.
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 1000F) 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 1000F and about 1040F, 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 effectively, and it will set code P0676 and illuminate a warning light as a result.
Where is the P0676 sensor located?
The image above shows the various components and construction of a typical modern glow plug.
Note though that although glow plugs typically screw into cylinder heads in much the same way as spark plugs on gasoline engines do, it is sometimes extremely challenging to gain access to the glow plugs on some applications. In some cases, it is necessary to remove not only cosmetic engine covers, but also to remove and/or disassemble major engine components such as manifolds and unrelated wiring, coolant and A/C hoses, vacuum lines, and even fuel rails to gain access to the glow plugs for testing or replacement purposes.
Therefore, be sure to consult reliable service information for the affected application to verify whether (or not) it is possible for non-professional mechanics to remove and replace the glow plugs on the affected application on a DIY basis.
What are the common causes of code P0676 ?
The common causes of code P0676 are largely similar across all applications, and could include one or more of the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connector(s) anywhere in the affected glow plug’ control and monitoring circuits
- Defective, broken, or dislodged glow plug current supply lead
- Defective glow plug; note that the heating coils in glow plugs form part of their control circuits, and a broken coil wire/element will therefore set an open circuit code
- Defective or malfunction glow plug control module
NOTE: While some resources define this code as “circuit malfunction”, this code is more properly associated with open or interrupted circuits. Issues like abnormal resistances or unsatisfactory performance of any particular glow plug will typically set codes other than P0676.
What are the symptoms of code P0676 ?
In some cases, there could be no discernible symptoms other than a stored trouble code and illuminated warning light present. However, in cases where discernible symptoms are present, these could include one or more of the following-
- Cranking times may be longer than usual in sub-zero ambient temperatures
- The idling may be slightly rough, or below the specified speed until the engine warms up enough for the control system to deactivate the glow plugs
- In some cases, some white or black smoke may be present immediately after the engine starts, although exhaust smoke will typically (or usually) disappear a few seconds after start-up
- Note that is extremely rare, if not unheard of for the failure of one glow plug to cause a no-start condition
- In severe cases where the glow plug tip might have disintegrated or broken off, the hard ceramic material in the glow plug can cause severe, if not always fatal damage to the affected cylinder’s valves, as well as to the piston and cylinder walls. Note that while these kinds of glow plug failures are rare, they can, and do occur during improper glow plug servicing/replacement procedures