P066B: Cylinder 1 Glow Plug Control Circuit High

What it means

The control line for the glow plug on cylinder 1 is open or shorted to supply - the plug cannot be heated.

At a glance

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

How it works

The engine controller commands the glow plug for cylinder 1 and checks whether its control circuit responds as expected. P066B sets when that circuit reads above its expected electrical range, which can indicate an open glow plug or wire, or a short to supply.

Common causes

Most likely first.

1. Open in plug or wiring
2. Connector pins corroded - high contact resistance
3. Supply wire pinched against B+ source

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. Save freeze-frame data and note whether the fault occurred during a cold start or glow plug operation.
2. With the engine cool, inspect the cylinder 1 glow plug connector and wiring for a disconnected plug, broken conductor, corrosion, or insulation damage.
3. Compare the cylinder 1 connection and harness routing with the other glow plug circuits to spot a local open or damaged wire.
4. Using the vehicle wiring diagram, test the cylinder 1 plug and control wire for continuity and for a short to supply against the manufacturer's specification.
5. If the plug and wiring pass, check the glow plug control module output and its power and ground connections.

Common fixes

• Reconnect or repair an open cylinder 1 glow plug wire.
• Repair corroded or damaged connector terminals.
• Replace the cylinder 1 glow plug if its element is open.
• Repair a wire shorted to supply or a faulty glow plug control module.

Can I keep driving?

The vehicle may run once started, but a failed cylinder 1 glow plug can cause difficult cold starts and white exhaust smoke during warm-up. Avoid depending on the vehicle in cold weather until the fault is repaired.