What Does It Mean?
This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may vary slightly by model.
P0684 indicates that the glow plug control module to PCM communication circuit has a problem in the range/performance aspect of communications. An error has occurred that prevents the powertrain control module (PCM) from communicating commands to the glow plug control module. The command is essentially an “on/off” signal.
Codes do not indicate a specific part of the system, just the area of failure. The glow plug circuit is relatively simple and can be diagnosed and repaired with little automotive knowledge, apart from a basic understanding of using a volt/ohmmeter.
What are glow plugs for?
Understanding their function requires a basic understanding of diesel engine operating principles.
Unlike a gasoline engine that needs a spark to ignite the fuel, a diesel engine relies on extremely high compression. When highly compressed, the air creates extreme heat. Diesel compresses the air in its cylinders to the point that the air reaches temperatures high enough to allow for spontaneous fuel ignition.
When a diesel engine block is cold, it is difficult to create enough heat through compression to ignite the fuel. The reason is that the cold engine block cools the air, causing a slower rise in temperature sufficient for starting.
When the vehicle’s powertrain control module (PCM) detects that the engine is cold via signals from oil and transmission temperature sensors, it turns on the glow plugs. The glow plugs heat to red-hot and transfer heat to the combustion chamber to assist in engine starting. They are on a timer and only operate for a few seconds. Any longer and they would burn out quickly.
How do they work?
When the PCM detects that the engine is cold, it signals a glow plug control module (GPCM). Once grounded, the GPCM grounds a glow plug solenoid (similar to a starter solenoid) on the valve cover.
The solenoid, in turn, sends power to a glow plug bus bar. The bus bar has a separate wire for each glow plug. The power travels to the glow plugs where they heat the cylinder to facilitate starting.
The GPCM is a timer and only stays activated for a few seconds. This is enough to start the engine, but at the same time, it protects the glow plugs from overheating due to prolonged activation.
Symptoms
Symptoms of a P0684 code can include:
The check engine light will be illuminated and the above codes will be set.
Little or no indication will be present if only one or two glow plugs have failed. If the engine is very cold, it may be slightly harder to start.
The engine may misfire until it has warmed up sufficiently.
If more than two glow plugs are malfunctioning, the engine will be very difficult to start.
Potential Causes
Causes
of this DTC can include:
Open or shorted wiring from the PCM to the GPCM, to the bus bar, or from the bus bar to the glow plug.
Faulty glow plug
Loose or corroded connections
GPCM has failed
Loose or corroded connections on the glow plug solenoid
Glow plug solenoid failure
Insufficient battery power to the solenoid
Code P0670 may accompany this code. This code indicates that the wiring harness from the GPCM to the solenoid is a problem.
Diagnostic and Repair Steps
Over the years, I have found this to be a common problem with diesel engines regardless of the manufacturer. Due to the high amperage required to operate the glow plugs and their tendency to burn out, I suggest starting with the most common issues.
The GPCM uses low amperage and, while possible, it is the least likely to fail. The solenoid is also rarely replaced. When dealing with high amperage, even the slightest play in a connection creates and sustains an arc, burning the connector.
Visually inspect the wiring between the PCM and the GPCM. Continue to the solenoid on the valve cover, and from the solenoid to the bus bar and on to the glow plugs. Look for loose or corroded connectors.
Disconnect the black and green electrical connector from the GPCM. Inspect the connector for bent pins and corrosion.
With the ohmmeter, check that each of the terminals is not shorted to ground. Repair the short if necessary.
Apply dielectric grease to the pins and reconnect the harness to the GPCM.
Inspect the positive battery connection and the GPCM connection on the glow plug solenoid. Ensure all wires are clean and tight.
Inspect the glow plug bus bar. Check the connection of each wire on the bus bar and ensure they are also clean and tight.
Remove the wire from the glow plug and check that it is not shorted.
Using the ohmmeter, probe the glow plug terminal with one lead and place the other on a ground. The glow plug is faulty if the resistance is not between 0.5 and 2.0 ohms. Check the resistance of the glow plug wiring to the bus bar. The resistance should also be between 0.5 and 2.0 ohms. If not, replace the wire.
If the above fails to resolve the issue, obtain a service manual and consult the page illustrating the glow plug circuit. Look at the color and pin number for the GPCM power supply and its power supply to the solenoid. Probe these terminals as indicated using the voltmeter.
If there is no power to the GPCM, the PCM has failed. If there is voltage to the GPCM, check the voltage between the GPCM and the solenoid. If there is no voltage at the solenoid, replace the GPCM.