Error Code P0683 is defined as Glow Plug Control Module to PCM Communication Circuit Code. This is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

When Error Code P0683 appears, this means the connections in the Glow Plug Control Module to the PCM (powertrain control module, also known as ECM or engine control module). Communication Circuit has been lost. Or a fault has occurred that prohibits the PCM from communicating its commands to the glow plug control module. Basically, this is an “on and off signal” issue.

This code doesn’t point to a specific part within the system, rather, it indicates an area of failure. The glow plug circuit is a simple system that can be easily diagnosed and repaired by anyone with little automotive background.

How glow plug works

Unlike gasoline engines that use spark to ignite the fuel, diesel engines use extremely high compression to create heat. The diesel compresses air within the cylinders and lets it reach a high temperature and ignite the fuel.

When a diesel engine block is cold, it will be difficult to create heat to ignite the fuel. This is because cold engine block cools the air, which slows the increase of temperature.

When the PCM senses that the engine is cold, it sends a signal to the oil and transmission temperature sensor to turn on the glow plugs. The glow plugs get red hot and transfers its heat to the combustion chamber to help raise the temperature and start the engine. All these works on a timer and takes place in a few seconds. Any longer and they would burn out quickly.

How they operate

When the PCM senses the engine is cold, it grounds the GPCM (glow plug control module), which then grounds a glow plug solenoid (similar to a starter solenoid) on the valve cover.

Then, the solenoid, in turn, sends power to the glow plug bus bar. The latter has a separate wire for each glow plug. Power then travels to the glow plug where they heat the cylinder and help with the startup.

The GPCM is a timer and is only active for a few seconds, enough to start the engine, but at the same time, protects the glow plugs from overheating.


Common Symptoms

The most common symptom for this code is the activation of the Check Engine light. However, in many cases, this may take up to eight cycles. Other noticeable symptoms include:

  • Poor acceleration
  • Stalling
  • Smoke coming from the exhaust
  • Increase in fuel consumption

Possible Causes

Typically, this code is caused by:

  • Electrical connection problem
  • Faulty glow plug control module
  • Connector or wiring problem in the CAN bus harness (open, short, or corroded)
  • Faulty PCM driver
  • Loose control module grounding strap
  • Broken ground wire
  • Damaged CAN bus circuits

How to Check

This error code is actually a common problem among diesel engines, regardless of the manufacturer. This is due to the high amperage needed to operate the glow plugs and their indication to burn out.  With this, it’s best to start with the common problems.

The GPCM uses low amperage, and though possible, is likely to fail. The solenoid is also seldom replaced, and when dealing with high amperage, even the least amount of looseness in the connection will cause an arc and burn out the connector.

Start by checking the wiring that runs from the PCM to the GPCM, and continue to the solenoid on the valve cover, all the way to the bus bar and down to the glow plugs. Check for corroded or loose connectors.

Disconnect the black and green electrical connector at the GPCM; check the connector for pushed out pins or signs of corrosion.

Using the ohmmeter, check each terminal for a short to ground, repair as needed.

Apply dielectric grease to the pins and then reconnect the harness to the GPCM.

Then, check the battery positive and GPCM connection on the glow plug solenoid. Make sure the wires are tight and clean.

Next, check the glow plug bus bar; inspect each wire connection and make sure they’re tight and clean as well.

Then, remove the wire from the glow plug, and then check for signs of short to ground.

Use the ohmmeter to inspect the glow plug terminal with one lead, and then place the other on ground. If it doesn’t go between 0.5 to 2.0 ohms, then the glow plug is defective.

Check the resistance in the wiring from the glow plug to the bus bar. Resistance must be in the 0.5 to 2.0 range. If not, then replace the wire.

If these steps for diagnosis still can’t pinpoint to the problem, then check with the service manual and turn to the page that shows the glow plug circuit. Check the color and pin number for power to the GPCM, and the power to the solenoid. Inspect the terminals as directed, using the voltmeter.

If there seems to be no power to the GPCM, that means the PCM has failed. If there’s no voltage to the GPCM, then check the voltage from the GPCM to the solenoid. If there’s still no voltage to the solenoid, then you must replace the GPCM.

How to Fix

Common repairs for this code include:

  • Repair or replacement of open, shorted, corroded, damaged, or disconnected wires, including fuses and components.
  • Check control module ground circuit, and test it with the battery ground for continuity, make sure no system grounds are defective or disconnected.
  • Check the diagram of the CAN bus system, and pin out value chart, and then test the controller connectors. What were the values set by the manufacturer? Compare these and then repair any damaged circuits.

Any codes related to the transmission, fuel injector, and engine misfires can be accompanied with this code, leaving rooms for misdiagnosis. Thus, it’s extremely important to get a proper diagnosis.

When diagnosing this code, they’re best to diagnose in the order they have been stored to avoid unsuccessful repairs. Freeze frame data shows the order the codes were set.

It’s best to replace defective wiring rather than trying to handle them individually in the wiring harnesses.