P0678: Cylinder 8 Glow Plug Circuit

What it means

An electrical fault is present in the glow plug circuit for cylinder 8. The glow plug control module cannot drive this plug correctly — open circuit, short, or out-of-range resistance — so cylinder 8 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 8.

At a glance

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

How it works

The cylinder 8 glow plug helps heat that cylinder for cold starting, and the glow plug control module controls its electrical supply. P0678 sets when the module detects an electrical fault in the cylinder 8 glow plug circuit, such as an open, short, or resistance outside the manufacturer’s specification. It directs diagnosis to cylinder 8’s plug, wiring, and module output.

Common causes

Most likely first.

1. Failed cylinder 8 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. Record freeze-frame data and scan for other glow-plug faults to determine whether cylinder 8 is the only affected circuit.
2. Confirm cylinder 8’s location using service information, then inspect its connector and lead for corrosion, a loose connection, or harness damage.
3. Isolate the circuit using the manufacturer’s procedure and compare the cylinder 8 glow plug measurement with the manufacturer’s specification.
4. Check the cylinder 8 lead for opens and shorts, including the connections at the glow plug module.
5. If the plug and wiring test good, test the module output assigned to cylinder 8 with the specified method.
6. Clear the code after repair and check for recurrence during a cold start.

Common fixes

• Reseat or clean the cylinder 8 glow plug connector.
• Repair damaged cylinder 8 wiring or connector terminals.
• Replace the cylinder 8 glow plug if it fails testing.
• Repair or replace the module’s cylinder 8 output if the plug and wiring are sound.

Can I keep driving?

Driving may be possible once the engine is warm, but a cold start can be difficult and may cause rough running or white exhaust smoke. Avoid extended cranking and repair the fault promptly if the engine continues to run unevenly.