HomeTrouble CodesP0679

P0679 — Cylinder 9 Glow Plug Circuitopen

Generic OBD-II · all makesPowertrain (engine & transmission)

The powertrain control module (PCM) has detected an open electrical circuit in the glow plug system for cylinder 9. This means the expected current flow through the glow plug is interrupted, usually due to a failed glow plug or break in the wiring. The code sets when the circuit continuity test fails during the glow plug activation sequence.

Symptoms

What causes it — most likely first

1. Failed glow plug

The heating element inside cylinder 9's glow plug has burned out or broken internally, creating an open circuit that prevents current flow.

2. Wiring or connector fault

A break, chafe, or corrosion in the harness between the glow plug and the control module interrupts the circuit path.

3. Glow plug control module failure

The relay or module responsible for supplying current to the glow plug bank may have an internal fault affecting the specific output for cylinder 9.

4. Poor ground connection

High resistance or an open ground path for the glow plug circuit prevents the completion of the electrical circuit.

5. PCM internal fault

Rarely, the PCM may incorrectly interpret a good circuit as open due to internal driver failure or software issues; this should only be considered after all wiring and components are verified good.

How it's diagnosed

  1. Inspect the glow plug connector and wiring for corrosion, damage, or looseness.
  2. Remove the glow plug and measure resistance across the terminals; an open reading confirms the glow plug has failed.
  3. Check for battery voltage at the glow plug harness connector during the glow plug activation cycle.
  4. Test continuity between the glow plug connector and the control module/PCM to identify wiring breaks.
  5. Verify the glow plug control module operation by testing output voltage to other cylinders for comparison.
  6. Only after confirming all wiring and components are good, consider PCM reprogramming or replacement as a last resort.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationSAE generic — identical on every make

Frequently asked

Can I drive with this code active?

Yes, but the engine will be difficult to start in cold weather and may produce excessive white smoke and run rough until warmed up.

Should I replace all glow plugs or just cylinder 9?

While only cylinder 9 is faulty, glow plugs typically age similarly; replacing the full set prevents future failures and repeated labor costs, though replacing only the failed plug is acceptable.

Is this caused by a bad computer (PCM)?

Almost never. This code indicates an open circuit in the wiring or the glow plug itself. PCM failure is extremely rare for this specific code and should only be considered after all wiring and components have been verified good.

What happens if I ignore this code?

The engine will start poorly in cold conditions, potentially causing battery drain from extended cranking and incomplete combustion that can foul the oil with fuel.