What Is P0671?
P0671 is a diesel-specific fault indicating a problem with the glow plug circuit on cylinder 1. The glow plug control module (GPCM) monitors each glow plug individually and logs a specific code when it detects a problem on a particular cylinder.
The fault is usually the glow plug itself - they are wear items that fail on high-mileage diesels. However, wiring faults between the module and the plug are also common, especially on vehicles where the glow plug wiring runs close to the exhaust manifold.
Common Symptoms
- Glow plug warning light on or flashing
- Hard starting in cold weather
- White smoke on cold start
- Rough idle until engine warms up
- Engine management light on
- Slight loss of power until engine reaches temperature
Common Causes
OBD Fault Code Plain English Guide
- 100 most common UK fault codes in plain English
- A severity rating for every code so you know what can wait
- Causes and next steps listed for each fault
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How to Diagnose P0671
Test Glow Plug Resistance
Disconnect the wiring from the cylinder 1 glow plug. Measure resistance between the plug terminal and earth (the engine block). A healthy glow plug reads 0.5–2 ohms. Open circuit (infinite) = failed plug. Near-zero resistance = internal short.
Check Wiring Continuity
With the plug disconnected, test continuity between the plug terminal and the glow plug module output for cylinder 1. No continuity = broken wire. Check where the harness routes near hot exhaust components.
Inspect Connector and Terminal
Check the glow plug connector for burnt or corroded terminals. Glow plug connectors run high current and can burn if not making proper contact. Clean or replace as needed.
Compare with Other Cylinders
If you have a multimeter, test all four glow plugs. Faulty plugs often fail in groups on high-mileage engines. Replacing only one plug when several are failing wastes time.
Remove and Inspect the Plug
If resistance tests confirm failure, remove the cylinder 1 plug. Soak the thread in penetrating oil first - glow plugs can seize after years of heat cycling. Use the correct glow plug socket and breaker bar.
Replace the Glow Plug
Fit the new glow plug with correct torque - typically 8–15 Nm. Do not overtighten as this can break the ceramic element or damage the thread. Clear the code and test cold start performance.
Verdict
Test resistance on the cylinder 1 plug first - a failed glow plug is the most common cause of P0671. If resistance is correct, check the wiring for damage near the exhaust.
P0672, P0673 and P0674 - Glow Plugs On Cylinders 2, 3 and 4
Glow plug circuit codes come as a family: P0671 is cylinder 1, and P0672, P0673 and P0674 are cylinders 2, 3 and 4. The diagnosis on this page covers all of them, because glow plugs are the same part doing the same job in each hole. What matters more than the cylinder number is the pattern: one plug code on a high mileage diesel is routine wear; codes on every cylinder at once point at the glow plug module or its supply feed instead (that story is covered under P0670 - glow plug module control circuit).
When one plug has failed, test all four while you have the multimeter out - they went into the engine on the same day and they age together. A healthy plug typically reads under an ohm or two to earth; a dead one reads open. It is common to find the coded plug dead and another one halfway there, and on a diesel that lives outdoors you will feel the difference on the first frosty morning.
One genuine warning for the DIY fit: glow plugs seize into aluminium heads, and a snapped plug can turn a £40 job into a machine shop visit. Crack them loose on a warm engine, use a proper torque figure rather than instinct, and if a plug will not move, soak it and walk away rather than leaning on the breaker bar.
