P0679 OBD-II Trouble Code: Cylinder 9 Glow Plug Circuit

diagnostic check engine light

P0679 code definition

The P0679 diagnostic trouble code is an indication that there is an issue with the cylinder 9 glow plug circuit.

Related Trouble Codes:

What the P0679 code means

Diesel engines are difficult to start when it is cold. A glow plug is located in each cylinder to provide heat is necessary to ignite the fuel when the engine is cold. When the Powertrain Control Module (PCM) determines that the engine is cold, it will use the glow plugs to assist in the combustion process. This enables diesel engines to run properly in all types of weather. The P0679 diagnostic trouble code is stored by the Powertrain Control Module (PCM) when the glow plug timer module activates the glow plugs and the cylinder #9 glow plug fails to heat up as it should.

What causes the P0679 code?

  • Faulty glow plugs
  • Faulty glow plug relays
  • Faulty glow plug module
  • Glow plug wiring that is frayed, broken, or shorted
  • Glow plug connectors that are damaged or corroded

What are the symptoms of the P0679 code?

How does a mechanic diagnose the P0679 code?

  • Diagnostic trouble codes should be diagnosed in the order that they appear in the powertrain control module (PCM).
  • Use an OBD-II scanner to collect all trouble codes that have been stored by the powertrain control module (PCM) as well as freeze frame data.
  • Inspect the glow plug wiring for frays, breaks, and shorted wiring.
  • Inspect the flow plug connectors for damage and/or corrosion.
  • Repair or replace glow plug wiring that is frayed, broken, or shorted.
  • Repair or replace glow plug connectors that are damaged or corroded.
  • Clear all diagnostic trouble codes from the powertrain control module (PCM) and complete a test drive to see if the P0679 diagnostic trouble code comes back.
  • If the P0679 trouble code does come back, test the glow plug connector using a volt ohmmeter.
  • If the voltage reading is not within the manufacturer’s predetermined range, the glow plug will need to be replaced.
  • After replacing the glow plug, clear all diagnostic trouble codes from the powertrain control module (PCM) and complete a test drive to see if the P0679 diagnostic trouble code comes back.
  • If the P0679 trouble code comes back, test the resistance level of the glow plug relay.
  • Compare the resistance level reading with the manufacturer’s specifications.
  • If the resistance level reading does not fall within the manufacturer’s specifications, the glow plug relay will need to be replaced.
  • Clear all diagnostic trouble codes from the powertrain control module (PCM) and complete a test drive to see if the P0679 diagnostic trouble code comes back.
  • If the P0679 trouble code comes back, test the glow plug module.
  • Compare the results of the test of the glow plug module with the manufacturer’s specifications.
  • If the results of the test do not coincide with the manufacturer’s specifications, the glow plug module will need to be replaced.
  • Clear all diagnostic trouble codes from the powertrain control module (PCM) and complete a test drive to see if the P0679 diagnostic trouble code comes back.
  • If the P0679 trouble code comes back, it is possible that the powertrain control module will need to be replaced. It is rare that for the powertrain control module (PCM) to malfunction, but it is possible.

Common mistakes when diagnosing the P0679 code

  • Failure to check the glow plug relay for proper operation
  • Failure to inspect glow plug connector for damage or corrosion
  • Failure to inspect glow plug wiring for frays, breaks or shorts

How serious is the P0679 code?

A diagnostic trouble code is normally considered serious when it causes driveability issues or a change in performance. While the P0679 code is stored in the Powertrain Control Module (PCM) the car or truck may be difficult to start, will not start at all, and may cause driveability issues. For this reason the P0679 diagnostic trouble code is considered serious and should be diagnosed and repaired as soon as possible.

What repairs can fix the P0679 code?

  • Replace faulty glow plugs
  • Replace glow plug relays
  • Replace glow plug module
  • Repair or replace glow plug wiring that is frayed, broken, or shorted
  • Repair or replace glow plug connectors that are damaged or corroded.

Additional comments for consideration regarding the P0679 code

During the diagnosis process, glow plug relays are commonly overlooked as the cause of a faulty glow plug. Glow plug wiring and connectors that are damaged and/or corroded are also commonly overlooked. It is important to check all glow plug relays, wiring and connectors before making a determination as to the cause of the P0679 diagnostic trouble code.

Need help with a P0679 code?

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The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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