Why does cylinder 4 cause more trouble?

It is usually the least accessible plug, often at the flywheel end near the exhaust. Heat, damp and infrequent disturbance make its plug and connector more prone to faults and harder to service.

Can I drive with P0674?

In mild weather, yes, as it mainly affects cold starting. In cold conditions starting will be harder and smokier, so it is best not to ignore it through winter.

Is it worth replacing all four plugs?

Often, on an older engine. If the hard-to-reach cylinder 4 plug has failed, doing the full set at the same time saves going back into an awkward job soon after.

What torque should the new plug be?

Most glow plugs call for around 8–15 Nm. Overtightening can crack the element or strip the thread in an alloy head, so use a torque wrench and the maker's figure.

Could the fault be the module, not the plug?

It can, but that is the least likely cause. Always confirm the plug, connector and wiring are good before condemning the glow plug control module, which is the expensive option.

Will P0674 affect my MOT?

The glow plug circuit itself is not an MOT test item, but the engine warning light being on, and any excess smoke from poor cold running, can both cause problems at test time.

P0674 - Glow Plug Circuit - Cylinder 4

What Is P0674?

P0674 is logged when the glow plug control module sees a circuit fault on cylinder 4. By measuring each plug's current draw during preheat, the module can tell that cylinder 4 specifically is not heating as it should, whether through the plug, its connector, or its wiring.

Cylinder 4 is usually the furthest plug from the timing belt, often at the flywheel end where access is tightest. That makes a seized plug or a corroded, hard-to-reach connector especially common reasons for P0674 on a higher-mileage diesel.

Common Symptoms

Glow plug warning light on or flashing
Poor cold starting
White exhaust smoke when cold
Rough idle until the engine warms
Engine management light
Reduced power from cold

Common Causes

Worn-out plug - the cylinder 4 element aged and failed, the leading cause.

Corroded connector - the awkward flywheel-end connector oxidising and losing contact.

Damaged wiring - a chafed or broken feed wire on the cylinder 4 branch.

Cracked element - a split plug tip causing an internal open or short.

Module drive fault - failure of the module output for cylinder 4, uncommon.

Loose terminal - the plug nut working loose, arcing and raising resistance.

How to Diagnose P0674

Identify the Flagged Plug

Use a scan tool to read per-cylinder glow plug status and confirm cylinder 4 is the one at fault, since it is normally the most awkward plug to get at.

Measure the Plug

Disconnect cylinder 4 and check resistance from terminal to block. A reading of roughly 0.5–2 ohms is good; an open reading means the element has failed.

Continuity to the Module

Test the feed wire from the cylinder 4 connector back to the module. Look for damage where the loom runs near the exhaust at the flywheel end of the engine.

Clean the Connector

Inspect the connector for corrosion and the nut for tightness. Connectors at this end of the engine collect dirt and damp, so cleaning often restores a good contact.

Check the Set

If access allows, ohm all four plugs. Cylinder 4 failing on a high-mileage engine usually means the others are ageing too, so plan accordingly.

Replace and Retest

Soak the thread, remove with the correct socket, and fit the new plug to torque. Clear the code, run a cold-start test, and confirm the glow light behaves.

Beware the Awkward PlugThe cylinder 4 plug is often the tightest to reach and the most likely to seize. Always work cold, never force a stuck plug, and use penetrating oil and patience - a sheared plug at the flywheel end is a difficult and costly extraction.

Verdict

Resistance-test the cylinder 4 glow plug first, since a worn plug is the most common cause of P0674. If it is sound, check the connector and the wiring at the flywheel end of the engine before looking at the control module.

Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.