P0674: Cylinder 4 Glow Plug Circuit

What it means

An electrical fault is present in the glow plug circuit for cylinder 4. The glow plug control module cannot drive this plug correctly — open circuit, short, or out-of-range resistance — so cylinder 4 is not pre-heated. On a cold start this causes hard starting, white smoke, and a misfire-like rough run until the engine warms. Causes: a failed glow plug, a broken supply lead to that plug, corrosion at the plug connector, or a faulty module output for cylinder 4.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The glow plug control module powers the glow plug in cylinder 4 to support ignition when the diesel engine is cold. P0674 sets when an electrical fault is detected in cylinder 4’s glow plug circuit, which may be caused by the plug, its wiring, or the module’s cylinder 4 output. It does not identify a fault in another cylinder’s branch.

Common causes

Most likely first.

1. Failed cylinder 4 glow plug
2. Open or shorted wiring to the plug
3. Corroded or loose connector
4. Glow plug control module output fault

Symptoms

• Glow plug warning light on (often orange coil symbol)
• Hard cold start or no start in cold weather
• White smoke from exhaust during warm-up

How to diagnose it

1. Capture freeze-frame data and check for other glow-plug codes to establish whether cylinder 4 is the only affected circuit.
2. Locate cylinder 4 from the vehicle’s service information and inspect its glow plug connector and wiring for looseness, corrosion, rubbing, or damage.
3. Isolate the circuit as directed by the manufacturer and measure the cylinder 4 glow plug against the manufacturer’s specification.
4. Test the cylinder 4 lead for continuity and shorts, checking both ends of the wire and the connector contacts.
5. If the plug and wiring pass, test the glow plug module’s cylinder 4 output using the manufacturer’s procedure.
6. Clear the code after repair and verify that it does not return during a cold-start check.

Common fixes

• Clean or reseat the cylinder 4 connector.
• Repair an open or shorted section of cylinder 4 wiring.
• Replace the cylinder 4 glow plug if its test result is outside specification.
• Repair or replace the module’s cylinder 4 output if the plug and wiring are confirmed good.

Can I keep driving?

The vehicle may run acceptably once warm, but cylinder 4 can be poorly heated during cold starts, causing rough running, white smoke, or difficult starting. Avoid prolonged cranking and fix the fault sooner if cold weather is expected or the engine remains uneven.