What does it mean?
This diagnostic code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1997 and newer), although specific repair steps may vary slightly by model.
This code relates to the device used by diesel engines to heat the cylinder head for a few seconds when attempting to start a cold engine, called a glow plug. A diesel engine relies entirely on the momentary high level of heat from compression to spontaneously ignite the fuel. The glow plug in cylinder #7 is faulty.
When a diesel engine is cold, the extremely high air temperature produced when the piston rises and compresses the air is quickly lost through heat transfer to the cold cylinder head. The solution comes in the form of a pencil-shaped preheater called a “glow plug.”
The glow plug is installed in the cylinder head very close to the point that initiates combustion, or the “hot spot.” This could be in the main chamber or the pre-chamber. When the engine management computer detects that the engine is cold using its oil and transmission sensors, it decides to assist the engine in starting by using the glow plugs.
A typical diesel engine glow plug:
Diesel engine glow plug
It grounds the glow plug timer module, which in turn grounds the glow plug relay that sends power to the glow plugs. The module powers the glow plugs. The module is usually integrated into the engine management computer, although some vehicles have it separately.
Activating it for too long will melt the glow plugs because they produce heat through high resistance and glow red when activated. This intense heat is quickly transferred to the cylinder head, allowing the combustion heat to maintain its temperature for the fraction of a second needed to ignite the incoming fuel for starting.
The code P0677 tells you that something in the glow plug circuit is faulty and that the glow plug in cylinder #7 is not heating. The entire circuit must be checked to locate the fault.
Note: If a P0770 anomaly code is present with this DTC, diagnose P0770 before diagnosing this code.
Symptoms
With a single glow plug failure, other than the check engine light coming on, symptoms will be minimal because the engine will start normally with a bad plug. In freezing conditions, you would be more likely to experience issues. The code is the primary way to identify such a problem.
The engine management computer (PCM) will set the code P0677
The engine will be hard to start or may not start at all in cold weather or when it has been sitting long enough for the block to cool.
Lack of power until the engine warms up sufficiently.
The engine may misfire due to the cylinder head temperature being colder than normal
The engine may hesitate during acceleration
No preheating period, i.e., the glow plug light does not go off.
Potential Causes
Causes
of this DTC may include:
Faulty glow plug in cylinder #7
Open or shorted glow plug circuit
Damaged wiring connector
Faulty glow plug control module
Diagnostic Steps
and Possible Solutions
You will need a digital volt ohmmeter (DVOM) for the full test. Continue testing until the problem is confirmed. You will also need a basic OBD code scanner to reset the computer and clear the code.
Check the glow plug in cylinder #7 by disconnecting the wire connector from the plug. Set the DVOM to ohms and place the red lead on the glow plug terminal and the black lead on a good ground. The range is 0.5 to 5.0 ohms (check your application’s specification by referring to a factory service manual). If it is out of range, replace the glow plug.
Check the resistance of the glow plug wire to the glow plug relay bus bar on the valve cover. Note that the relay (looks like a starter relay) has a large gauge wire leading to a bus bar where all the glow plug wires are attached. Test the glow plug wire for number one by placing the red lead on the number one wire at the bus bar and the black lead on the glow plug side. Again, 0.5 to 5.0 ohms, with 5 ohms being the highest resistance. If it is higher, replace the glow plug wire from the bus bar. Also note that these wires from the bus bar to the plugs are fuse links. Reconnect the wires.
Check the same wires for looseness, cracks, or missing insulation. Connect the code scanner to the OBD port under the dashboard and turn the key to the “on” position with the engine off. Clear the codes.