Is P0673 a serious fault?

Not in terms of safety, but it does affect cold starting and emissions. It is best fixed before winter, when a weak glow system makes starting much harder and produces more smoke.

Why is cylinder 3 harder to reach?

On many transverse diesels the rear cylinders sit close to the bulkhead with less working room, so the plug and its wiring take more effort to access than the front ones.

Can a bad earth cause P0673?

Glow plugs earth through the cylinder head, so a poor engine-to-body earth can affect readings. It is worth checking the main earth straps if the plug and wiring both test fine.

Do I need to code a new glow plug?

No. Glow plugs are not coded. You fit the correct part, clear the stored code, and confirm the module no longer reports a fault on cylinder 3.

How long does the job take?

If access is reasonable, replacing one plug takes around 30–60 minutes. A seized plug or one needing manifold removal can extend that considerably.

Should the glow light go out after repair?

Yes. Once the faulty plug or wiring is fixed and the code cleared, the glow plug warning light should behave normally and stay off after the preheat cycle.

P0673 - Glow Plug Circuit - Cylinder 3

What Is P0673?

P0673 is stored when the glow plug control module detects a circuit fault on cylinder 3. The module watches the current drawn by each plug as it heats, so it can isolate the fault to a single cylinder rather than the whole glow system.

Cylinder 3 sits further back along the engine, closer to the bulkhead on most transverse diesels, so its wiring is exposed to heat and is more fiddly to reach. The plug itself is still the likeliest fault, but a tired connector or chafed feed wire runs a close second.

Common Symptoms

Glow plug light illuminated or flashing
Difficult cold starting
White smoke from cold
Lumpy running until the engine warms through
Engine management light on
Minor power loss when cold

Common Causes

Failed glow plug - the cylinder 3 element worn out, the most frequent cause on an older diesel.

Open feed wire - a break in the wiring to cylinder 3, often heat-related near the bulkhead.

Connector corrosion - oxidised terminals raising resistance on the cylinder 3 plug.

Internal short - a cracked element shorting inside the plug.

Control module output - loss of the module's cylinder 3 drive, relatively rare.

Loose retaining nut - the terminal nut backing off and arcing under load.

How to Diagnose P0673

Confirm With Live Data

Read each glow plug's reported status on a scan tool and check that cylinder 3 is the one flagged. This avoids disturbing the more accessible plugs unnecessarily.

Ohm the Cylinder 3 Plug

Disconnect and measure resistance from the plug terminal to earth. Around 0.5–2 ohms is healthy; infinite resistance confirms the element has burned through.

Test Wiring to the Module

Check continuity from the cylinder 3 connector to the module. Because this wire often runs near the bulkhead and exhaust, inspect closely for heat damage and chafing.

Examine Terminal and Connector

Look for corrosion, burning, or a loose nut on the cylinder 3 plug. Clean the contact and retighten to spec; a poor high-current joint behaves just like a failed plug.

Cross-Check the Others

Measure the remaining plugs while you are there. Group failure is common, and knowing the condition of all four helps you decide whether to replace one or the set.

Fit the New Plug

Soak the threads first, remove carefully with the proper socket, and torque the new plug correctly. Clear the fault and check the glow light and cold-start behaviour.

Take Your Time on RemovalCylinder 3 can be tucked toward the bulkhead and the plug may be seized. Work only on a cold engine, use plenty of penetrating oil, and stop if it will not move freely - shearing a plug here is awkward and expensive to put right.

Verdict

Start at the cylinder 3 glow plug with a resistance test, as a worn plug is the usual cause of P0673. If it tests fine, follow the wiring toward the bulkhead and check the connector before condemning 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.