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.
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
How Your ECU Detects This
Technical sensor logic and voltage thresholds
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 |
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.
When to See a Professional Mechanic
Not all fault codes are safe to DIY
Code P0671 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
How to Clear Code P0671
What happens after you fix the fault
Once the fault is repaired, P0671 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.
✅ Safe to Clear When
- Fault has been diagnosed and repaired
- You want to confirm the repair worked
- Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
- Preparing for an emissions/PUC test
- Root cause is still undiagnosed
- Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.