Comprehensive diagnostic guide for P0679: Cylinder 9 Glow Plug Circuit Malfunction OBD-II

Disclaimer on sources and mapping

• The OBD-II framework and the nature of diagnostic trouble codes (DTCs) are described generally in Wikipedia's OBD-II articles, including how codes are used and what powertrain codes encompass. This guide uses those general concepts to frame a P0679-specific diagnostic workflow.
• The provided open-source entry labeled indicates a mapping to a glow plug/heater for cylinder 9. This is used here to anchor the code to a cylinder-2 glow plug circuit.
• No NHTSA complaint dataset is provided with these sources; probabilities below are and typical automotive diagnostics practice when precise complaint data isn't available.

What This Code Means

• Based on the mapping in the provided open-source entry, P0679 corresponds to a Cylinder 9 Glow Plug Circuit Malfunction. In practical terms, this is a fault detected in the electrical circuit that powers the glow plug for cylinder 9 (glow heater for that cylinder), which may involve the glow plug itself, wiring, connectors, fuses/relays, and the PCM driver that powers the glow plug circuit. The PCM uses glow plugs to aid starting in cold conditions, so a fault can manifest as hard starting in cold weather or a MIL illumination with a stored circuit fault.

Symptoms

• MIL illumination with DTC P0679 stored in the PCM.
• Difficulty starting the engine, particularly in cold weather or at first start when glow plugs are expected to heat.
• Rough or uneven idle on cold start until the engine reaches operating temperature.
• Frequent or intermittent long cranking before the engine starts.
  Note: These symptoms reflect the typical role of glow plugs in aiding cold starts and are consistent with glow plug circuit faults in general, though the exact symptom set may vary by vehicle and engine layout. (General glow plug behavior; not all symptoms appear on every vehicle)

Diagnostic Approach

Phase 0 - Safety and preparation

• Ensure the vehicle is on a level surface, the parking brake is set, and the engine is cool before handling glow plug wiring or electrical components.
• Gather needed tools: OBD-II scan tool, DVOM (digital volt/ohm meter), test light or current-sensing probe, back-probing tools, service manual or wiring diagrams for cylinder 9 circuit, appropriate fuses/relays chart, and a known-good glow plug for comparison if replacing.
• Have the service manual handy for cylinder-to-cylinder spark/glow plug mapping and for the specific engine's glow plug voltage, resistance ranges, and drive logic.

Phase 1 - Verify and contextualize the fault

• Use an OBD-II scan tool to confirm P0679 is current (not history) and note any freeze-frame data or any accompanying codes (for example misfire codes like P0309 for cylinder 9 or codes in the circuits related to the glow plug driver).
• Check for related codes that often accompany glow plug issues, such as misfire codes, harness faults, or PCM output faults if the PCM reports a driver problem.

Phase 2 - Visual and mechanical inspection

• Inspect the cylinder 9 glow plug wiring harness, connector, and electrical harness for damage, corrosion, pin corrosion, or loose connections.
• Inspect the fuse(s) and relay related to the glow plug circuit (and any PCM relay or electronic switch controlling the glow plug circuit). Look for blown fuses, melted wires, or corroded terminals.
• Inspect the glow plug for cylinder 9 for signs of mechanical damage, oil contamination on the tip, or carbon/soot buildup that could indicate abnormal operation or combustion issues in the cylinder.
• Check for signs of harness chafing or routing issues that could cause intermittent open/short circuits.

Phase 3 - Electrical testing (circuit-level tests)

• Glow plug resistance test:
  • With the engine cool and the glow plug connector disconnected, measure the resistance of the Cylinder 9 glow plug (from terminal to ground or as specified by the service manual). A very high resistance or open circuit indicates a failed glow plug. Compare against OEM specification.
• Power supply and ground checks:
  • With the ignition or cold-start condition engaged, verify that battery voltage (or the source voltage for the glow plug circuit) appears at the glow plug supply terminal when the PCM is commanding heat. If there is no supply voltage, backtrace to the fuse/relay, wiring harness, and PCM output.
  • Verify a solid ground path for the glow plug circuit. A poor or intermittent ground can mimic a fault.
• Control circuit (PCM driver) checks:
  • Using a scope or a logical test tool, observe the PCM's output to the glow plug circuit (the glow plug driver). Confirm there is a controlled current/voltage when glow is commanded. If the PCM output is missing or erratic, the fault may be in the PCM driver, wiring, or a higher-level control condition.
• Wiring and connector integrity:
  • Perform a thorough continuity check for the glow plug circuit wiring from the PCM/driver to the glow plug connector. Look for shorts to ground or to power, opens, or high resistance due to poor connections.
  • Inspect not only the two-wire circuit to the glow plug but also any intermediate connectors or splices. A single bad connector can trigger the DTC.

Phase 4 - Component-level checks and decisive tests

• If the glow plug resistance test shows a faulty glow plug:
  • Replace Cylinder 9 glow plug with an OEM-equivalent part. Re-test the circuit after replacement.
• If the glow plug itself tests fine but the circuit voltage is erratic or absent:
  • Inspect and repair wiring harness and connectors. Ensure proper engagement and no exposed wires. Clean or reseat connectors as needed. Apply dielectric compound where appropriate if the vehicle design calls for it.
• If the PCM output is not driving the glow plug as commanded:
  • Confirm there are no conflicting fault codes that could cause the PCM to disable glow plug output (for example, a higher-priority safety fault). If necessary, reflash/update the PCM per OEM guidelines after repairs if warranted.
  • Consider PCM-related faults if all wiring and the glow plug show within spec in bench tests; PCM faults are less common but possible.
• If fuses/relays are found faulty:
  • Replace the faulty fuse or actuator (relay) as per OEM specification.

Phase 5 - Post-repair verification

• Clear the DTCs and perform a cold-start test to verify the glow plug circuit for cylinder 9 operates correctly. Monitor for persistent P0679 and confirm there are no related codes.
• Observe engine starting performance in cold conditions to confirm improvement. If the vehicle has a cold-start issue that previously triggered P0679, a successful repair should resolve it or reduce glow-related starting issues.
• If the code reappears, revisit the wiring, connectors, and the possibility of cross-talk or a secondary fault (e.g., a neighboring glow plug circuit affected by harness damage, or multiple glow plug circuits in a multi-cylinder engine layout).

Probable-cause breakdown and rough probability guidance (field experience perspective)

• Faulty Cylinder 9 Glow Plug element (open/high resistance): ~40-50%
  • Most direct cause when the glow plug is the sensed fault (and the driver may not see supply when commanded).
• Wiring harness or connector fault in the Cylinder 9 glow plug circuit (open/short, corrosion, loose connection): ~20-30%
  • Common due to environmental exposure, routing, and multiple mating connectors.
• Faulty glow plug driver in the PCM or incorrect PCM output (driver fault or control condition not activating): ~10-20%
  • Less common than a physical glow plug or wiring fault, but occurs in some vehicles, especially with PCM aging or software/logic issues.
• Blown fuse or failed relay supplying the glow plug circuit: ~5-10%
  • A simple, observable supply issue that prevents the circuit from energizing.
• Wiring damage due to external factors (chafing, pin damage) or short to ground/voltage: ~5%
  • Additional contributors when harness damage exists.

Notes on mapping and cross-vehicle considerations

• Cylinder numbering and glow plug configuration can vary by engine family and maker. Always verify cylinder-to-glow-plug mapping with the specific vehicle's service manual or OEM wiring diagram before performing cylinder-specific testing or replacement. The cylinder-9 glow plug mapping referenced in the provided open-source entry should be treated as a guide aligned with that specific mapping; verify against your vehicle.
• If your vehicle uses a different numbering scheme or a different set of glow plugs (e.g., V-shaped engines with dual banks), consult the OEM documentation to confirm which cylinder corresponds to the glow plug circuit you're testing.

Safety Considerations

• Glow plugs operate at low voltage (12V) but can heat rapidly and cause burns; avoid touching the glow plug tip while the system is energized or recently energized.
• Disconnect the battery or disconnect the glow plug circuit power when performing measurements that could cause short circuits or arcing.
• Use insulated tools and proper PPE when handling wiring and electrical tests.
• Verify all checks with the vehicle in a safe state, and avoid misinterpreting a failing PCM output as a simple wiring fault; ensure you've thoroughly tested the wiring and the glow plug before drawing a PCM fault conclusion.

Documentation

• Record: VIN, engine family, cylinder mapping (which cylinder is cylinder 9 in this engine), observed DTCs, freeze-frame data, test results (glow plug resistance value, circuit voltage when commanded, continuity results), and the exact replacements performed (glow plug, fuse/relay, wiring harness, PCM update if applicable).
• After repair, perform a controlled test drive and cold-start test to confirm the issue is resolved and the P0679 code does not reappear.

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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