P0678 – Cylinder 8 Glow Plug Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Cylinder 8

Definition source: SAE J2012/J2012DA (industry standard)

P0678 is a powertrain diagnostic trouble code that indicates a fault in the cylinder 8 glow plug circuit. In diesel applications, the glow plug system is used to help heat the combustion chamber during cold starts and, on some platforms, to stabilize combustion during warm-up. This code does not, by itself, prove the glow plug is bad; it only confirms the control module detected an electrical circuit issue associated with the cylinder 8 glow plug path. The exact monitoring strategy, enable conditions, and what the module considers a fault can vary by vehicle, so always verify component locations, wiring routing, and test specifications using the correct service information for the vehicle you are working on.

What Does P0678 Mean?

P0678 means the control module has detected a circuit fault related to the glow plug circuit assigned to cylinder 8. “Glow plug circuit” refers to the electrical path that delivers and controls current to the glow plug, typically including the glow plug itself, the harness/connectors, and the driver/control element that switches power to that plug. SAE J2012 defines the structure and naming of DTCs; for this code, the official definition is specifically limited to the cylinder 8 glow plug circuit, without implying a particular failure mode beyond an electrical circuit problem that must be confirmed by testing.

Quick Reference

  • Subsystem: Cylinder 8 glow plug electrical circuit (glow plug, wiring, connectors, and control/driver path).
  • Common triggers: Open circuit, short to ground or power, excessive resistance, poor connector contact, or a driver/control issue affecting the cylinder 8 glow plug circuit.
  • Likely root-cause buckets: Wiring/connector faults, glow plug internal electrical fault, power/ground distribution issue, glow plug control module/driver fault, or less commonly a control module logic issue (varies by vehicle).
  • Severity: Usually most noticeable during cold starts and warm-up; may range from mild to hard-start/no-start depending on temperature and system design.
  • First checks: Confirm cylinder numbering, inspect cylinder 8 connector and harness routing, check for corrosion/heat damage, verify related fuses/feeds, and scan for companion glow plug or power supply codes.
  • Common mistakes: Replacing the glow plug without verifying the circuit, misidentifying cylinder 8, overlooking shared power/ground faults, or ignoring connector pin fit and corrosion.

Theory of Operation

The glow plug system uses electrically heated elements installed in each cylinder’s pre-combustion area (design varies by engine) to raise local temperature for improved ignition quality, especially during cold starts. A control module (or integrated driver within another module) commands glow plug operation based on inputs such as temperature and engine state. Current is delivered through dedicated wiring and connectors, often with a shared power feed and individual controlled outputs.

The module monitors circuit integrity by evaluating commanded state versus observed electrical behavior (strategy varies by vehicle). If the cylinder 8 circuit does not respond as expected, or if the circuit shows signs consistent with an electrical fault (such as an open, short, or abnormal resistance effect), the module flags P0678 and may disable or limit operation for that channel to protect the driver.

Symptoms

  • Hard start: Extended cranking, especially in colder ambient temperatures.
  • Rough idle: Uneven running immediately after start that improves as the engine warms.
  • Misfire-like shake: Temporary vibration or stumbling during cold operation due to incomplete combustion in one cylinder.
  • Smoke: Increased exhaust smoke at cold start or during warm-up (combustion stability affected).
  • Warning lamp: Check engine light illuminated; some vehicles may also indicate a glow plug/system warning.
  • Reduced warm-up quality: Slower stabilization of idle speed and throttle response right after starting.

Common Causes

  • Open circuit or high resistance in the cylinder 8 glow plug control wire between the control module/relay and the glow plug
  • Poor connector contact at the cylinder 8 glow plug (corrosion, spread terminals, poor pin fit, loose locking)
  • Damaged wiring harness near heat sources or sharp edges causing intermittent opens or shorts
  • Glow plug internal electrical failure (open element or abnormal resistance) affecting circuit feedback
  • Shared power feed issue to the glow plug system (fuse, fusible link, relay feed, or distribution point problem) that shows up on one branch depending on design
  • Ground path problem (engine ground strap, ground splice, or ground eyelet condition) increasing circuit resistance
  • Glow plug control module/relay output fault for the cylinder 8 channel (if individually controlled; varies by vehicle)
  • Water intrusion or contamination in connectors leading to intermittent circuit faults during temperature or vibration changes

Diagnosis Steps

Useful tools include a scan tool capable of reading freeze-frame and running live data, a digital multimeter, and a wiring diagram/service information for connector pinouts and specifications. A basic test light can help with quick power checks where appropriate. Back-probing tools, terminal inspection picks, and supplies for cleaning/repairing terminals are also helpful.

  1. Confirm DTC P0678 is present and record freeze-frame data and any related glow plug or power supply DTCs. Clear codes and see if P0678 returns immediately or only after a cold start (behavior varies by vehicle).
  2. Use service information to identify cylinder numbering and the physical location of cylinder 8, then locate the cylinder 8 glow plug and its connector to avoid testing the wrong circuit.
  3. Perform a visual inspection of the cylinder 8 glow plug harness routing, connector lock, terminal condition, and nearby heat shielding. Look for chafing, melted insulation, oil saturation, or signs of water intrusion.
  4. With the system powered down as required by service information, disconnect the cylinder 8 glow plug connector and inspect both sides for corrosion, looseness, bent pins, or backed-out terminals. Repair terminal fit issues before deeper testing.
  5. Check the glow plug itself electrically using the method specified in service information (typically a resistance/continuity check). If it is open or clearly out of specification, replace it and recheck for code return.
  6. Verify the circuit has the required power feed during a commanded preheat event (or during the controller’s self-test). Use the scan tool to command the glow plug system if supported; otherwise follow the cold-start/preheat procedure described in service information.
  7. Test the control wire(s) between the controller/relay and cylinder 8 for continuity and for short-to-ground/short-to-power conditions with the circuit isolated. Any open, high resistance, or unintended continuity indicates wiring or connector damage that must be repaired.
  8. Perform voltage-drop testing across the power and ground paths that serve the cylinder 8 glow plug circuit while the circuit is commanded on (where design allows). Excessive drop indicates unwanted resistance in connectors, splices, fuses, relay contacts, or grounds.
  9. Conduct a wiggle test while monitoring live data, commanded state, and/or measured voltage at the cylinder 8 connector. Move the harness and connectors along the routing to reproduce intermittent opens, poor pin fit, or internal breaks.
  10. If wiring, power/ground paths, and the glow plug test good, evaluate the glow plug control module/relay output for the cylinder 8 channel (as applicable). Confirm correct inputs (power, ground, control command) to the module and verify the output behaves correctly under command before considering replacement.

Professional tip: When P0678 is intermittent, prioritize tests that load the circuit (commanded on) and capture evidence over time. Logging live data during a cold soak start, combined with voltage-drop checks at the connector and a careful wiggle test, often reveals a high-resistance terminal or broken conductor that looks fine during static continuity checks.

Need wiring diagrams and factory-style repair steps?

Powertrain faults often require exact wiring diagrams, connector pinouts, and guided test steps. A repair manual can help you confirm the cause before replacing parts.

Factory repair manual access for P0678

Check repair manual access

Possible Fixes & Repair Costs

Repair costs for P0678 can vary widely because the fault is circuit-related and the true cause may be the glow plug itself, wiring/connectors, the power feed/ground path, or the control hardware. Labor time depends on access to cylinder 8 and harness routing.

  • Repair or replace damaged wiring in the cylinder 8 glow plug circuit after confirming the fault with testing
  • Clean, tighten, and correctly seat connectors; address corrosion, terminal spread, or poor pin fit at the glow plug, harness, or controller connection
  • Restore proper power feed and ground integrity to the glow plug circuit (including repairing high-resistance connections found by voltage-drop testing)
  • Replace the cylinder 8 glow plug only after verifying it fails resistance/continuity checks or fails an appropriate functional test per service information
  • Repair harness routing issues causing chafing/heat damage; secure the loom to prevent repeat failures
  • If applicable for the platform, replace the glow plug control device only after verifying inputs/outputs and confirming the circuit external to it is not the cause
  • Clear the DTC and confirm the fix by running the appropriate enable conditions and rechecking for pending/confirmed codes

Can I Still Drive With P0678?

In many cases, P0678 does not create an immediate safety hazard once the engine is running, but it can cause hard starting, rough running during warm-up, and increased emissions, especially in cold conditions. If the vehicle will not start reliably, stalls, enters a reduced-power mode, or any critical warning indicators appear, do not drive; diagnose the cylinder 8 glow plug circuit and verify proper operation using service information for your vehicle.

What Happens If You Ignore P0678?

Ignoring P0678 may lead to progressively worse cold-start performance, longer cranking, rough idle during warm-up, and ongoing malfunction indicator illumination. Repeated difficult starts can increase wear on the starting/charging system and may mask additional faults. The underlying circuit issue can also worsen over time if caused by corrosion, overheating, or harness chafing.

Compare nearby cylinder glow trouble codes with similar definitions, fault patterns, and diagnostic paths.

  • P0682 – Cylinder 12 Glow Plug Circuit
  • P0681 – Cylinder 11 Glow Plug Circuit
  • P0680 – Cylinder 10 Glow Plug Circuit
  • P0679 – Cylinder 9 Glow Plug Circuit
  • P0677 – Cylinder 7 Glow Plug Circuit
  • P0676 – Cylinder 6 Glow Plug Circuit

Key Takeaways

  • P0678 indicates a fault in the cylinder 8 glow plug circuit, not a guaranteed failed glow plug
  • Most confirmed causes fall into wiring/connector issues, power/ground integrity problems, or a faulty glow plug after testing
  • Use test-driven diagnostics (continuity, voltage-drop, and functional checks) before replacing parts
  • Driveability impact is often most noticeable during cold starts and warm-up
  • Verifying the repair requires clearing the code and confirming the monitor passes under the correct conditions

Vehicles Commonly Affected by P0678

  • Vehicles equipped with diesel engines that use glow plugs for cold-start assistance
  • Engines with an individual glow plug circuit per cylinder monitored by a control module
  • Platforms using a dedicated glow plug controller or integrated control electronics
  • High-mileage vehicles where harness insulation, connectors, and terminals may degrade
  • Vehicles operated in cold climates with frequent glow plug activation cycles
  • Applications with tight engine bay packaging where heat and vibration stress wiring
  • Vehicles exposed to moisture, road salt, or corrosive environments affecting connectors
  • Engines where cylinder-numbering orientation can lead to misidentification without service information

FAQ

Does P0678 mean the cylinder 8 glow plug is bad?

No. P0678 means the cylinder 8 glow plug circuit is faulted. The glow plug may be defective, but the same code can be caused by wiring damage, connector problems, poor power/ground, or a control-side issue. Testing is required to confirm the failed component.

Can a wiring problem set P0678 even if the glow plug is new?

Yes. A new glow plug will not prevent P0678 if the circuit has an open, high resistance, corrosion at terminals, a poor ground path, or a damaged power feed. Circuit integrity checks and voltage-drop testing help isolate these issues.

Why is cylinder numbering important for P0678?

P0678 is specific to cylinder 8. Cylinder numbering varies by vehicle, so you should confirm cylinder 8 location using service information before testing or replacing components to avoid diagnosing the wrong cylinder circuit.

Will clearing the code fix P0678?

Clearing the code only resets stored fault information; it does not correct the underlying circuit problem. If the fault is still present, P0678 may return as a pending or confirmed code after the monitor runs under the proper conditions.

What is the best confirmation test after repairs for P0678?

After repairing the verified cause, clear the DTC, then confirm the glow plug circuit functions correctly using the appropriate functional test or command (varies by vehicle), and recheck for pending/confirmed codes after the enable conditions are met.

Always confirm the final repair by verifying cylinder 8 glow plug circuit integrity and monitor completion using the correct service information and test procedure for the specific vehicle.