P0679 – Cylinder 9 Glow Plug Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Cylinder 9

Definition source: SAE J2012/J2012DA (industry standard)

P0679 is a powertrain diagnostic trouble code indicating a fault in the Cylinder 9 glow plug circuit. Glow plugs are used on many diesel engines to help warm the combustion chambers for reliable cold starts and stable early idle. When the control module detects an electrical circuit problem affecting the glow plug for cylinder 9, it stores P0679 and may illuminate the malfunction indicator lamp. DTC behavior and the exact monitoring logic can vary by vehicle, so confirm component locations, wiring routing, and test specifications in the appropriate service information before testing or replacing parts.

What Does P0679 Mean?

P0679 means the vehicle’s control module has detected a fault in the electrical circuit associated with the glow plug for cylinder 9. The code is tied specifically to the circuit (wiring, connectors, and driver/control path) for that one cylinder’s glow plug, not to a general glow plug system concern. While SAE J2012 defines how DTCs are structured and categorized, the definition here remains strictly “Cylinder 9 Glow Plug Circuit,” so diagnosis should focus on verifying circuit integrity, proper power and ground delivery, and correct control of the cylinder 9 glow plug during commanded preheat/afterglow events.

Quick Reference

• Subsystem: Cylinder 9 glow plug circuit (glow plug, connector, wiring, and the control/driver path that commands it)
• Common triggers: Open circuit, shorted wiring, poor terminal contact, failed glow plug element, or a driver/control fault that prevents expected circuit response
• Likely root-cause buckets: Wiring/connector issues; glow plug (actuator) fault; power/ground supply problem; glow plug control driver/module fault (varies by vehicle)
• Severity: Typically mild to moderate; cold-start difficulty and rough running are more likely than immediate safety risk, but severity increases in cold conditions
• First checks: Confirm cylinder numbering for “cylinder 9,” inspect connectors and harness routing, check for corrosion/loose pins, and verify the glow plug circuit responds to commanded activation
• Common mistakes: Replacing multiple glow plugs without testing the cylinder 9 circuit, misidentifying cylinder 9, or overlooking a connector/terminal fit issue near heat sources

Theory of Operation

In a glow plug system, each cylinder’s glow plug is an electrically heated element that raises local combustion chamber temperature to support ignition during cold starts and to smooth combustion during warm-up. The control module (or a dedicated glow plug controller, depending on vehicle design) commands glow plug operation based on operating conditions such as temperature and start status. Current through the glow plug is switched by an internal driver or external controller and delivered through a dedicated circuit to each plug.

The module determines whether the cylinder 9 glow plug circuit is healthy by monitoring electrical behavior when the circuit is commanded on or off. Depending on design, it may infer circuit integrity from measured current flow, voltage behavior at the driver, or feedback signals. If the response is inconsistent with an intact circuit (for example, due to an open, short, excessive resistance, or a driver issue), it flags a circuit fault and stores P0679.

Symptoms

• Cold-start difficulty: Longer crank time or harder starting, especially in low ambient temperatures
• Rough idle: Uneven idle or shaking during the first moments after startup
• White smoke: Increased white exhaust smoke during cold start due to incomplete combustion (conditions vary by vehicle)
• Misfire-like feel: Noticeable stumble until the engine warms, potentially more apparent under light load right after starting
• Reduced smoothness when cold: Hesitation or poor response during warm-up that improves as temperature rises
• MIL on: Malfunction indicator lamp illuminated with P0679 stored

Common Causes

• Open circuit in the cylinder 9 glow plug control wire between the control module and the glow plug
• High resistance in the cylinder 9 glow plug circuit from corrosion, damaged insulation, or partially broken conductors
• Poor terminal fit, backed-out pin, or contaminated connector at the glow plug, harness junction, or control module connection
• Short to ground in the cylinder 9 glow plug circuit wiring (chafing against metal, pinched harness)
• Short to power in the cylinder 9 glow plug circuit wiring (contact with a powered feed, harness damage)
• Faulty glow plug at cylinder 9 (internal open/short or abnormal resistance that disrupts circuit operation)
• Glow plug control module or driver channel fault affecting the cylinder 9 circuit (internal switching/monitoring issue)
• Power feed or ground fault shared by the glow plug system that causes one circuit to fail monitoring under load (varies by vehicle)

Diagnosis Steps

Tools that help: a scan tool capable of reading DTCs, freeze-frame data, and relevant live data; a digital multimeter; and basic back-probing or breakout leads. A test light may help for quick power/ground checks where appropriate. Use the correct wiring diagram and connector views for your vehicle to identify cylinder 9, circuit routing, and module pins.

1. Confirm P0679 is present and record freeze-frame data and any companion glow plug or power supply codes. If multiple glow plug circuit codes are present, diagnose shared power/ground or module supply issues first (varies by vehicle).
2. Verify cylinder numbering and the physical location of cylinder 9 using service information. Do not assume bank/cylinder location, as layouts vary by vehicle.
3. Clear codes, then command or observe a glow plug event (as supported by the scan tool and vehicle). Monitor live data related to glow plug activation/feedback if available, and see whether P0679 resets immediately, only during activation, or intermittently.
4. Perform a careful visual inspection of the cylinder 9 glow plug connector and nearby harness routing. Look for melted plastic, heat damage, oil saturation, corrosion, chafing, or a harness pinched under brackets. Repair obvious wiring damage before deeper testing.
5. Connector integrity check: with the connector unplugged (and ignition state per service info), inspect terminals for spread pins, poor retention, backing-out, or contamination. Correct terminal fit issues and clean/repair as required.
6. Continuity and open-circuit check: using the wiring diagram, test the cylinder 9 glow plug circuit for continuity end-to-end (module-to-glow plug) and verify there is not an open. If continuity is inconsistent, perform a wiggle test on the harness while watching the meter to locate an intermittent open.
7. Short-to-ground check: with the circuit isolated as appropriate (connectors unplugged per service info), test the cylinder 9 control wire for unintended continuity to chassis ground. If present, locate chafing points or crushed sections and repair the harness.
8. Short-to-power check: verify the cylinder 9 control wire is not unintentionally connected to a powered feed. If a short-to-power is found, inspect harness routing near power distribution points and repair the damaged section.
9. Glow plug check: test the cylinder 9 glow plug electrically per service information (method varies by vehicle). If it tests out of specification or shows an internal open/short condition, replace it and recheck the circuit.
10. Voltage-drop testing under load: during a commanded glow plug event (or equivalent test mode), perform voltage-drop tests across the cylinder 9 circuit path (power side and ground/return path as applicable) to identify excessive resistance at connectors, splices, or terminals. Compare results across cylinders only if the vehicle uses comparable circuit architecture.
11. Module/driver evaluation: if wiring and the glow plug test good, verify the glow plug control module has correct power and ground supplies and that the cylinder 9 driver/feedback behaves consistently with other channels (where applicable). If the driver does not switch/monitor correctly and all external causes are eliminated, the module may be the fault.
12. Final verification: after repairs, clear DTCs and repeat the activation/monitoring conditions that originally set P0679. Road test if needed and re-scan to confirm the monitor completes without the code returning.

Professional tip: If P0679 is intermittent, prioritize live-data logging during cold-start conditions and combine it with a harness wiggle test at known movement/heat points. Many circuit faults only appear when current demand is high, so a clean continuity test with no load may miss a high-resistance connection; voltage-drop testing during activation is often the fastest way to pinpoint the weak link.

Possible Fixes & Repair Costs

Repair costs for P0679 can vary widely because the root cause may be a simple connection issue or a component failure, and access to the affected cylinder’s circuit can change labor time. Confirm the fault with testing before replacing parts.

• Repair damaged wiring to the cylinder 9 glow plug circuit (open, chafed insulation, corrosion, heat damage) and restore proper routing/retention
• Clean, repair, or replace poor-fitting, corroded, or contaminated connectors/terminals at the glow plug, harness junctions, and control/module connections
• Replace the cylinder 9 glow plug only after confirming it fails resistance/continuity checks per service information
• Repair power feed or ground path issues that supply the glow plug system (fuses, fusible links, relays, distribution points, ground eyelets) as applicable by vehicle
• Replace a failed glow plug control device (driver/module/relay, varies by vehicle) only after verifying the circuit and load are correct
• Address module-related issues only after all circuit integrity tests pass and the fault can be reproduced/verified per service information

Can I Still Drive With P0679?

In many cases, a cylinder 9 glow plug circuit fault mainly affects cold starting and initial smoothness rather than immediate safety. However, if you experience a no-start, extended cranking, severe rough running, reduced power, warning messages that affect drivability, or any signs of electrical overheating (burning smell, smoke), do not continue driving; stop safely and diagnose the circuit. When symptoms are mild, drive cautiously and plan repairs soon, especially in cold conditions.

What Happens If You Ignore P0679?

Ignoring P0679 can lead to progressively harder cold starts, longer crank times, increased misfire-like roughness during warm-up, and potentially higher emissions during initial operation. Continued operation with an unresolved circuit fault can also stress related electrical components and may worsen harness or connector damage if overheating or arcing is present.

Related Cylinder Glow Codes

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
• P0678 – Cylinder 8 Glow Plug Circuit
• P0677 – Cylinder 7 Glow Plug Circuit
• P0676 – Cylinder 6 Glow Plug Circuit

Key Takeaways

• P0679 indicates a fault in the cylinder 9 glow plug circuit, not a guaranteed failed glow plug.
• Most confirmed causes are circuit-related: opens, poor connections, corrosion, or power/ground feed problems.
• Verify the fault with circuit tests before replacing components, especially the glow plug or control device.
• Symptoms are often most noticeable during cold starts and early warm-up.
• Address signs of electrical overheating immediately and avoid driving until inspected.

Vehicles Commonly Affected by P0679

• Diesel engines equipped with individual cylinder glow plugs and electronic monitoring
• Engines that use a dedicated glow plug control device to drive and monitor each plug circuit
• Applications with glow plug harnesses routed near high-heat sources where insulation can degrade
• Vehicles operated in cold climates where glow plug use is frequent and faults become more apparent
• Higher-mileage vehicles with increased likelihood of terminal fretting, corrosion, or connector tension loss
• Vehicles with recent engine work where a glow plug connector or harness may be left loose or pinched
• Applications exposed to moisture, road salt, or chemical contamination that can corrode terminals
• Vehicles with battery/charging issues that can contribute to repeated starting stress and circuit complaints

FAQ

Does P0679 mean the cylinder 9 glow plug is bad?

No. P0679 means the cylinder 9 glow plug circuit is detected as faulty. The glow plug itself can be the cause, but the issue may also be an open circuit, poor connector contact, wiring damage, or a driver/control problem. Testing is required to confirm the failed part.

What is the most common reason a cylinder glow plug circuit code sets?

Common reasons include an open circuit from a damaged wire, a loose or corroded connector at the glow plug, or terminal tension problems that create intermittent contact. A failed glow plug is also possible, but it should be verified with resistance/continuity checks.

Can a blown fuse or bad power feed cause P0679?

Yes, depending on how the system is designed. If the cylinder 9 glow plug circuit loses its required feed or ground path, the control module may detect an abnormal circuit condition and set P0679. Use service information to identify the correct fuses, relays, and distribution points for the glow plug system.

Will clearing the code fix the problem?

Clearing the code only resets stored fault information. If the underlying circuit problem remains, P0679 will typically return after the next self-test or operating conditions that run the glow plug monitor (often during a cold start or preheat event). Clear codes only after repairs and verification testing.

Why does the code seem worse in cold weather?

Glow plugs are used more aggressively in colder conditions, so the system runs its checks more often and the engine is more sensitive to reduced preheating on a single cylinder. A marginal connection or partially open circuit may be intermittent and become more noticeable when demand and preheat time increase.

If P0679 returns after repairs, recheck connector pin fit, perform a wiggle test while monitoring circuit status, and confirm the harness is not contacting hot or sharp surfaces that can recreate an open-circuit condition.