Comprehensive diagnostic guide for OBD-II code P0678

Code in focus: P0678 - Cylinder 8 Glow Plug Circuit Fault (Glow plug/Preheater circuit for cylinder 8)

What This Code Means

• P0678 is categorized as a powertrain (P-code) diagnostic trouble code in OBD-II systems (Powertrain Codes section of the OBD-II references). These codes are generated by the vehicle's onboard diagnostic system to indicate faults in the engine, transmission, or related components that affect emissions or performance.
• An open-source listing for this code is titled Aquecedor/vela aquecedora do cilindro 8, which translates to "glow heater/plug of cylinder 8," aligning with the meaning of P0678 as a cylinder-8 glow plug circuit fault. This supports the typical interpretation that the issue resides in the glow plug circuit for cylinder 8.
• Emissions testing context is relevant: DTCs, including P-codes, are part of the diagnostic framework used to satisfy emissions regulations and readiness, so a persistent glow plug circuit fault may affect cold-start behavior and emissions performance if applicable to the engine family.

Symptoms

• MIL (malfunction indicator lamp) illuminated or flashing in some cases.
• Difficult cold-starts or extended crank time, especially in cold weather.
• Rough idle or intermittent misfire indications around startup if the glow plug circuit is intermittent.
• Hard starting in engines that rely on glow plug preheating to initiate combustion, or noticeable lack of heat in cylinder 8 during pre-heat cycles.
• In some vehicles, no obvious drivability issue other than the MIL, if the system self-corrects or the fault is intermittent.

What to consider first (scope and limits)

• P0678 specifically points to the glow plug circuit for cylinder 8. The root cause is commonly in the glow plug itself, the wiring harness/connector, the glow plug relay or power supply to the circuit, or the engine control module (PCM/ECU) interpreting the circuit fault. The exact configuration (number of cylinders with glow plugs, whether glow plugs are used on your gasoline engine or diesel preheater system) varies by engine family, but the code name consistently references cylinder 8.
• Remember that DTCs are part of the emissions & performance diagnostics framework; once the code is cleared, the fault may reappear if the underlying issue is not repaired.

Probable Causes

• Faulty glow plug in cylinder 8: ~45%
• Open/short in glow plug wiring or connector to cylinder 8: ~25%
• Faulty glow plug relay or power supply (fuse/relay circuit for the glow plug circuit): ~15%
• PCM/ECU fault or misinterpretation by the controller (sensing or driver circuit issue): ~5%
• Ancillary issues (corroded grounds, short to ground, wiring harness damage, oil contamination on plug area): ~10%

Notes:

• These percentages are approximate field-based estimates, intended to guide diagnostic priority when no vehicle-specific data is available. They are not drawn from a single official dataset and may vary by vehicle make/model.

Step-by-Step Diagnosis

1) Verify and gather data

• Confirm P0678 is current/stored in the ECU and pull freeze-frame data (engine temperature, RPM, load, etc.) at the time of fault.
• Check for related cylinder misfire codes or other glow-plug-related codes (if your vehicle stores additional codes such as P067x family variants, or P035x (spark/coil) codes in gasoline engines).
• Note vehicle make/model/year, engine type, and whether the engine uses actual glow plugs (diesel or specific gasoline-engine preheaters) for cylinder 8.

2) Visual inspection

• Inspect wiring to cylinder 8 glow plug: look for damaged insulation, chafing, sleeve wear, oil/grease contamination, or exposed conductors.
• Inspect the glow plug connector/socket for corrosion, bent pins, or poor seating.
• Check the glow plug relay (or controller output) and associated fuses for signs of overheating, corrosion, or failure.
• Inspect the cylinder 8 glow plug area for oil leaks or contamination that could affect electrical contact.

3) Electrical tests (primary circuit checks)

• Glow plug resistance (cold engine, with battery disconnected if required by procedure)
  • Measure resistance of glow plug in cylinder 8; compare to the manufacturer's specification (specs vary; consult the service manual for your engine). An open circuit (infinite resistance) or well-out-of-spec resistance indicates a faulty glow plug.
• Power supply and control signal
  • With ignition on (engine off) verify that the glow plug receives appropriate voltage when commanded by the PCM (you may need a service manual or OEM scan tool capability to see the glow cycle).
  • Check for voltage drop or intermittent supply along the wiring harness to cylinder 8.
• Relay and fuse check
  • Test the glow plug relay coil resistance and the primary/secondary contacts (relay often switches battery voltage to the glow plug circuit). Check for sticking contacts or failed relay.
  • Inspect related fuses; replace if open or blown and check for short to ground in the circuit.
• Ground integrity
  • Verify a solid ground path for the glow plug circuit. Poor grounding can mimic a high-resistance glow plug or a weak supply condition.

4) Functional/active testing (when tools allow)

• Glow plug activation test
  • If the scan tool supports glow plug/ preheat circuit tests, perform a glow cycle test to verify that cylinder 8 plug heats within the expected duration and current range.
• Current draw test (advanced)
  • With proper precautions, measure the current drawn by the glow plug circuit during activation. A significantly low or high current relative to spec can indicate the plug or wiring condition.

5) If electrical tests indicate a faulty component

• Replace the glow plug in cylinder 8 if resistance and function tests point to a failed element.
• Repair/replace wiring harness or connectors if open/short/poor contact is found.
• Replace the glow plug relay or corresponding control hardware if power-supply issues are observed.
• If all electrical checks pass but the fault persists, consider PCM/ECU fault or software/firmware issues; this is less common and usually a last-resort validation step.

6) After repair

• Clear the P0678 code (and any related codes) and perform a complete drive cycle per OEM recommendations to verify the repair.
• Monitor live data with a scan tool to ensure cylinder 8 glow plug circuit activates correctly during cold-start or preheat as designed.
• Recheck for any recurring fault codes after a reasonable drive cycle, including cold-start tests.

Safety Considerations

• Disconnect the battery as needed before performing electrical tests on glow plug circuits to avoid accidental short circuits or arcing.
• Glow plugs can get extremely hot after engine operation. Allow time to cool before handling or removal.
• When applying power testing or bench testing glow plugs, use proper current-limiting and follow vehicle-specific service procedures to avoid damage or injury.
• Use eye protection and gloves when working around high-temperature components and electrical systems.

Repair tips and practical notes

• The most frequent fix for P0678 is a failed cylinder 8 glow plug or an intermittent wiring/connector issue. Prioritize testing the glow plug itself and the wiring harness/connector before replacing the PCM.
• Do not assume the glow plug is at fault solely based on resistance reading; corroborate with voltage supply, activation data, and current draw during a glow cycle.
• If the vehicle uses a shared glow plug circuit with multiple plugs, verify the entire circuit's integrity; sometimes a common feed or ground fault can affect more than one plug.
• When replacing glow plugs on engines that require torque specs and anti-seize procedures, follow the manufacturer service manual to avoid damaging threads or affecting preheating performance.

Correlation with the sources

• Diagnostic Trouble Codes and Powertrain Codes context: The diagnostic framework for P-codes (including P0678) is described in the OBD-II sections on Diagnostic Trouble Codes and Powertrain Codes, which explain that codes arise from onboard diagnostic monitoring and relate to engine/transmission systems.
• Cylinder 8 glow plug reference: The open-source listing titled Aquecedor/vela aquecedora do cilindro 8 provides a direct cue that the code relates to the glow plug for cylinder 8, aligning with the cylinder-specific glow plug fault interpretation.
• Emissions testing framework: The Emissions Testing section under OBD-II notes that the diagnostic framework ties into emissions readiness, reinforcing the importance of resolving glow plug circuit faults as they can affect cold-start efficiency and compliance.

Documentation

• P0678 indicates a fault in the cylinder 8 glow plug circuit. The likely causes are a bad glow plug, wiring/connector issues, a faulty glow plug relay or power supply, or a PCM fault. The most common fix is replacing the cylinder 8 glow plug or repairing the circuit wiring/connector. After repairs, clear the code and verify via a drive cycle to ensure the glow plug circuit activates properly during cold starts.

References (supporting sources)

• Open Source listing: Aquecedor/vela aquecedora do cilindro 8. Provides a direct cue that the code maps to the glow plug for cylinder 8 (glow plug circuit fault).

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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