How do I know if a glow plug is working?

Test it with a multimeter — a good plug reads 0.5–2 ohms. You can also test by applying power directly and checking the current draw, or feel if the tip gets hot within 10 seconds.

Should I replace all glow plugs at once?

If one has failed on a high-mileage diesel, the others are likely close to failure. Replacing all four at once is more cost-effective than returning for each one individually.

How long does it take to replace a glow plug?

On accessible engines, 30–60 minutes for a set of four. On heavily packaged engines, several hours may be needed if the inlet manifold must be removed.

What happens if I ignore a faulty glow plug?

Cold starting becomes difficult, particularly below 5°C. White smoke and rough running when cold will increase. Repeated hard starts also wear the starter motor and battery faster.

How much do glow plugs cost?

Individual glow plugs cost £8–£30 each depending on vehicle. A full set of four with fitting typically costs £80–£200 at an independent garage.

Can a faulty glow plug cause a misfire code?

Yes, indirectly. A completely failed glow plug can result in poor combustion on that cylinder when cold, sometimes logging a misfire code alongside the glow plug circuit code.

P0671 — Fault Code | Mr Auto Fixer

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

Failed cylinder 1 glow plug — worn element, most common cause

Open circuit in the wiring between the glow plug module and cylinder 1 plug

Short circuit in the glow plug wiring — short to earth or battery

Corroded glow plug connector or terminal

Failed glow plug control module output for cylinder 1

Cracked glow plug body causing internal short

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.

Seized Glow PlugsNever attempt to remove a glow plug from a hot engine. Always allow the engine to cool completely. If the plug feels very tight, stop immediately and apply more penetrating oil. Forcing a seized plug will shear it, requiring specialist extraction tools and significant additional cost.

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.

P0671 — Fault Code