AutoFaultCodes.com
OBD-II fault code definitions, symptoms, causes, and step-by-step diagnostics.
Standardized generic DTCComputer & output circuits

P0678 — Cylinder 8 Glow Plug Circuit

P0678 is the standardized powertrain DTC for “Cylinder 8 Glow Plug Circuit.” The code identifies the monitored condition; it does not by itself prove which part failed.

Standardized generic definition
Cylinder 8 Glow Plug Circuit
DTC category
Computer & output circuits
Diagnostic priority
Moderate. Glow-plug faults primarily affect cold starting and cold combustion on compression-ignition engines, but circuit faults should still be repaired.
Copy code

What P0678 means

P0678 is the standardized powertrain DTC for “Cylinder 8 Glow Plug Circuit.” The code identifies the monitored condition; it does not by itself prove which part failed.

Start with the stored data and the simplest external checks before replacing the component named in the code.

Possible symptoms

  • Hard cold starting
  • Longer cranking in low temperatures
  • White smoke or rough cold idle
  • Glow-plug warning or Check Engine Light

Causes to test

These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.

  • Failed glow plug
  • Open/high-resistance plug circuit or connector
  • Glow-plug control module/relay fault
  • Power-supply, fuse/fusible-link, or ground issue
  • Module communication/control problem

Step-by-step diagnostic workflow

  1. Confirm battery state and cold-start conditions before testing.
  2. Inspect the affected plug/module circuit and high-current connections for heat damage or corrosion.
  3. Measure plug resistance/current using manufacturer specifications; do not assume all glow plugs use the same resistance/current.
  4. Verify module power, ground, and commanded output.
  5. For cylinder-specific codes, compare the affected circuit with known-good cylinders when the procedure permits.
  6. Clear codes only after repair, then complete an appropriate drive cycle and verify the code and symptoms do not return.
Preserve evidence: EPA OBD guidance emphasizes that stored DTCs and freeze-frame data are important to proper diagnosis. Save them before clearing codes.

Scan data worth checking

  • Glow-plug command and feedback where supported
  • Coolant/intake temperature at cold start
  • Battery voltage during pre-glow/crank
  • Cylinder-specific glow-plug status

Severity and drivability

Moderate. Glow-plug faults primarily affect cold starting and cold combustion on compression-ignition engines, but circuit faults should still be repaired.

Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.

Common diagnostic mistakes

  • Applying battery voltage directly to a modern low-voltage glow plug
  • Replacing all plugs without checking the control module and supply circuit
  • Testing only when the engine is warm and the system is not being commanded

Related generic codes

P0677 — Cylinder 7 Glow Plug CircuitP0679 — Cylinder 9 Glow Plug CircuitP0676 — Cylinder 6 Glow Plug CircuitP0680 — Cylinder 10 Glow Plug CircuitP0675 — Cylinder 5 Glow Plug CircuitP0681 — Cylinder 11 Glow Plug Circuit

Source and accuracy notes

Generic DTC structure and standardized definitions are governed by SAE J2012/J2012DA. Actual enable criteria, thresholds, circuit pinouts, test values, software conditions, and repair procedures vary by vehicle. Use current OEM service information for the exact year/engine/calibration and check applicable manufacturer technical service bulletins. See the site methodology and primary-reference policy.