P0671: Cylinder 1 Glow Plug Circuit

What it means

An electrical fault is present in the glow plug circuit for cylinder 1. The glow plug control module cannot drive this plug correctly — open circuit, short, or out-of-range resistance — so cylinder 1 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 1.

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 supplies the glow plug fitted to cylinder 1 to help that cylinder ignite fuel during a cold start. P0671 sets when the module detects an electrical problem in the cylinder 1 glow plug circuit, such as an open, short, or resistance outside the manufacturer’s specification. It identifies cylinder 1’s branch, not a general fault affecting every glow plug.

Common causes

Most likely first.

1. Glow plug element open
2. Wiring or connector 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. Record freeze-frame data and check for other cylinder-specific glow-plug codes to see whether the fault is limited to cylinder 1 or shared across several outputs.
2. Locate cylinder 1 using the vehicle’s service information, then inspect its glow plug connector and lead for looseness, corrosion, heat damage, or rubbing.
3. With the circuit safely de-energized as specified by the manufacturer, compare cylinder 1 glow plug resistance with the manufacturer’s specification and with other plugs if appropriate.
4. Check the cylinder 1 lead for continuity and shorts, paying attention to the connector and harness routing near the plug.
5. If the plug and wiring pass, use a scan tool or the prescribed circuit test to check the glow plug module’s cylinder 1 output.
6. Replace or further test components only after confirming the fault remains on the cylinder 1 circuit.

Common fixes

• Reconnect or clean the cylinder 1 glow plug connector.
• Repair a damaged cylinder 1 glow plug lead or connector.
• Replace the cylinder 1 glow plug if its measured condition is outside specification.
• Repair or replace the module’s cylinder 1 output if the plug and wiring test good.

Can I keep driving?

A vehicle that runs normally once warm may be drivable, but cylinder 1 may misfire during cold starts, produce white smoke, or become difficult to start in low temperatures. Limit repeated cold-start attempts and address it promptly if starting worsens or rough running continues.