P0678
Cylinder 8 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 8 isn't heating up properly before the diesel engine starts, like a lighter that won't ignite. The engine control unit detected a circuit problem preventing the glow plug from receiving the correct electrical signal.

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Symptoms You May Notice
3 known symptoms for this code
Hard starting, especially in cold weather
White smoke from exhaust on startup
Rough idle after cold start
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors voltage and current flowing through the cylinder 8 glow plug relay circuit during the glow plug preheat cycle. It detects open circuits, shorts to ground, or excessive resistance that prevents proper heating coil activation. The fault triggers when measured resistance or voltage deviates beyond acceptable limits.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Glow plug circuit voltage 10.5-14.5V during preheat Below 8V or open circuit detected
Glow plug coil resistance 0.5-2.0 ohms Above 5 ohms or infinite resistance
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Glow plug relay
Test the relay with a multimeter; if faulty, swap it with an identical relay from another cylinder to confirm before replacement.
2
Glow plug (cylinder 8)
Remove the glow plug using a deep socket and test it with a multimeter for continuity and resistance in the 0.5-2.0 ohm range.
3
Wiring harness and connectors
Inspect the glow plug connector and wiring for corrosion, burnt terminals, or loose connections and repair or replace as needed.