P0677: Cylinder 7 Glow Plug Circuit

What it means

An electrical fault is present in the glow plug circuit for cylinder 7. The glow plug control module cannot drive this plug correctly — open circuit, short, or out-of-range resistance — so cylinder 7 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 7.

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 energizes the cylinder 7 glow plug to help that cylinder ignite fuel in a cold diesel engine. P0677 sets when the module detects an electrical fault in the cylinder 7 circuit, including an open, short, or resistance outside the manufacturer’s specification. It identifies the cylinder 7 branch rather than a general glow plug fault.

Common causes

Most likely first.

1. Failed cylinder 7 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 whether other glow-plug codes are stored, especially codes for neighboring cylinders.
2. Use service information to identify cylinder 7 and inspect its glow plug connector and individual wire for corrosion, looseness, chafing, or heat damage.
3. Following the specified isolation procedure, measure the cylinder 7 glow plug and compare it with the manufacturer’s specification.
4. Test the cylinder 7 lead for continuity and shorts, checking its terminals at both the plug and control module.
5. If the plug and wiring pass, test the glow plug module’s cylinder 7 output according to the manufacturer’s procedure.
6. Clear the code after repair and verify the circuit during a cold-start check.

Common fixes

• Clean or reseat the cylinder 7 glow plug connector.
• Repair an open, short, or damaged terminal in the cylinder 7 wiring.
• Replace the cylinder 7 glow plug if it does not meet the specified test result.
• Repair or replace the module’s cylinder 7 output if the plug and wiring pass.

Can I keep driving?

The engine may be usable once warm, but cylinder 7 may not preheat correctly, making cold starts rough or difficult and increasing white smoke during warm-up. Avoid repeated start attempts and arrange repair if the engine runs unevenly or starting deteriorates.