P0670: Glow Plug Module Control Circuit

What it means

The control circuit between the engine controller and the glow plug control module reports a fault. The ECU can no longer reliably command the module that drives the glow plugs, so cold-start pre-heating may not run. On a diesel this causes hard starting, white smoke, and rough running until warm. Causes: open or shorted wiring between ECU and glow plug module, a failed module, or a power/ground fault at the module.

At a glance

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

How it works

The engine controller sends commands to the glow plug control module, which switches power to the engine’s glow plugs before and sometimes after a cold start. P0670 sets when the controller detects a fault in its control circuit to that module, so it may not be able to request preheating reliably. The code points to the module’s control path; it does not by itself identify a particular glow plug.

Common causes

Most likely first.

1. Glow plug control module faulty
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 glow-plug or low-voltage codes, since they can help distinguish a control-signal problem from a shared power supply fault.
2. Inspect the control-module connector and harness between the engine controller and glow plug module for looseness, corrosion, chafing, or damaged wires.
3. Check the module’s power and ground connections for poor contact or excessive voltage drop, following the manufacturer’s test procedure.
4. With a suitable scan tool, compare the commanded glow-plug operation with the module’s reported status, if available.
5. Test continuity and shorts in the control circuit between the engine controller and module using the wiring diagram and manufacturer’s specification.
6. If the wiring, power, and ground pass, test the module and its control input before considering an engine-controller fault.

Common fixes

• Clean and secure a loose or corroded module connector.
• Repair damaged wiring in the module control circuit.
• Restore a faulty power or ground connection at the glow plug module.
• Replace the glow plug control module if its inputs and supplies are correct but it fails testing.

Can I keep driving?

If the engine starts and runs smoothly, short trips may be possible, but cold starting can become unreliable and emissions can rise. Avoid repeated cranking if it struggles to start; arrange repair before cold weather or if the warning light flashes or the engine runs poorly.