P0679
Cylinder 9 Glow Plug Circuit
Powertrain Ignition System Glow Plug Circuit 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The glow plug for cylinder 9 isn't heating up properly before a cold diesel start, like a lighter that won't ignite. The engine's computer detected a circuit problem preventing the glow plug from receiving or maintaining the correct electrical signal.

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Symptoms You May Notice
3 known symptoms for this code
Hard starting in cold weather
White smoke on startup
Glow plug warning light on dashboard
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors voltage and current draw through the cylinder 9 glow plug circuit during the glow plug preheat cycle. It detects open circuits, shorts to ground, or excessive resistance that prevents proper heating. The control module expects a specific voltage drop and amperage within defined timing windows.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Glow Plug Circuit Voltage 11-14V during preheat cycle Below 8V or no voltage detected
Circuit Resistance 0.5-2 ohms per plug Above 10 ohms or infinite (open circuit)
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Battery terminals and cables
Clean corrosion from battery terminals and check cable connections to ensure proper voltage supply to the glow plug control module.
2
Glow plug wiring harness connector
Inspect and reseat the cylinder 9 glow plug connector, checking for bent pins, corrosion, or loose fitment.
3
Glow plug (cylinder 9)
Replace the faulty glow plug after confirming voltage reaches the connector; test with a multimeter before replacement.