P0671
Cylinder 1 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 1 isn't working properly, which helps diesel engines start in cold weather. It's like a heating element that won't turn on when needed.

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Symptoms You May Notice
3 known symptoms for this code
Hard starting or no start, especially in cold weather
White smoke from exhaust on startup
Engine running rough or misfiring on cylinder 1
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors the glow plug circuit voltage and current draw during the glow plug pre-heat phase before cranking. It expects a specific voltage drop and resistance when the relay activates the glow plug element. If voltage is absent, circuit resistance is too high, or the plug draws no current, a fault is detected.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Glow Plug Circuit Voltage 11-14V during glow phase Below 8V or open circuit
Glow Plug Current Draw 8-15 amps per plug Zero amps or excessive resistance
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Glow plug for cylinder 1
Remove the old glow plug and install a new OEM or quality replacement; torque to spec (typically 12-15 ft-lb).
2
Glow plug relay
Test or replace the glow plug relay if the circuit voltage is missing at the plug connector.
3
Engine harness and connectors
Inspect and clean corroded connectors and wiring between the relay, ECU, and cylinder 1 glow plug.