What does it mean when the glow plug light flashes?

A flashing glow plug light usually signals a fault in the glow plug system or its control circuit and should be diagnosed promptly. Glow plugs heat the combustion chamber to aid cold starts, and a flashing indicator is a warning rather than normal operation.

What the flashing glow plug light means

The glow plug indicator is there to alert you to problems in the starting preheat system or its control logic. When it flashes, it can indicate a fault in one or more of these areas:

• Faulty glow plugs (one or more) failing to heat properly.


• Failure in the glow plug relay or wiring to the plugs.


• Low battery voltage or charging problems affecting glow plug operation.


• Engine control module (ECM/PCM) or glow plug controller faults.


• Diagnostic trouble codes indicating glow plug or cylinder-specific heater faults (examples include P0380 and P0671–P0679).

In any case, a flashing light is not something to ignore. It often correlates with difficulties starting the engine in cold weather or rough running, and continuing to drive without diagnosis can risk damage to starting components or continued misfiring.

How to diagnose and respond

Here is a practical checklist to understand and address a flashing glow plug light. The items assume a diesel engine but many steps apply to modern diesels with glow plug systems.

1. Read diagnostic trouble codes with an OBD-II scanner to identify glow plug or related circuit faults.


2. Check the battery and alternator health. Glow plugs rely on adequate voltage; test for proper voltage (engine off around 12.6V; charging system around 13.5–14.5V when running).


3. Inspect glow plug wiring, connectors, and fuses for corrosion, loose connections, or damaged insulation. Replace or repair as needed.


4. Test individual glow plugs and the glow plug relay. A bad plug typically shows high resistance; a failed relay will not engage the heater circuit.


5. If codes point to cylinder-specific plugs, test or replace the affected glow plugs; in some engines, replacing one can be straightforward, in others it may require more extensive service.


6. Consider professional service if the light persists after basic checks or if you are not comfortable performing electrical diagnostics.

Once the diagnosis is complete, follow the recommended remedial actions, which could range from cleaning and reseating connectors to replacing glow plugs and related components. If you are unsure, seek professional help to avoid damage or dangerous starting conditions.

When to seek immediate service

Seek immediate service if the engine is hard to start in freezing weather, if there is significant white smoke on startup, or if the engine runs rough or misfires continuously while the light is flashing. Persistent issues can prevent proper starting and cause increased emissions or engine damage.

What to expect at the shop

At a service visit, technicians typically: verify the fault codes, test battery and charging systems, inspect the glow plug harness and relays, perform resistance checks on glow plugs, and replace any failing plugs or components as needed. They may also inspect wiring and the ECU program for faults and update software if indicated.

Summary

A flashing glow plug light signals a fault in the glow plug system or its control circuitry and should be diagnosed promptly to prevent starting problems or engine damage. Use diagnostic codes to guide inspection, check electrical health, and seek professional help if you are not comfortable performing electrical diagnostics. Addressing the issue early can restore reliable cold-weather starting and overall engine performance.

What to do if the glow plug light is flashing?

A flashing glow plug light usually indicates a glow plug circuit fault. Start by testing each glow plug for resistance using a multimeter; faulty plugs show infinite or very high resistance. Next, inspect the glow plug relay and fuse for damage or corrosion. Replace any defective glow plugs or relay.

Is it safe to drive a car with a glow plug light flashing?

It is not recommended to drive with a flashing glow plug light. Continuing to drive with this warning can lead to poor engine performance, reduced fuel efficiency, and potential engine damage. It's best to diagnose and fix the issue as soon as possible.

Can I drive with a faulty glow plug?

Yes, you can technically drive a car with faulty glow plugs, especially if the issue is minor and the weather is warm, as they are primarily used for startup and warming up the engine. However, driving with bad glow plugs is not recommended because it can lead to engine performance issues, reduced fuel efficiency, and potential long-term engine damage, and you risk your car not starting again. It is best to get the issue fixed as soon as possible. 
Risks of driving with bad glow plugs

• Starting problems: As more glow plugs fail, it becomes increasingly difficult to start the engine, and in cold weather, the car may not start at all. 
• Reduced performance: The engine may run roughly, hesitate, or have less power, which can be a safety hazard, especially on busy roads. 
• Lower fuel efficiency: Incomplete combustion from faulty glow plugs can lead to the engine using more fuel than it should. 
• Increased emissions: Poor combustion can also result in higher emissions. 
• Potential for further damage: Ignoring the problem can lead to more expensive repairs, such as carbon buildup in the cylinders or damage to emissions control equipment. 

This video explains the symptoms of bad glow plugs, including their impact on engine performance: 1mStudent LessonYouTube · Apr 26, 2024
What to do if your glow plugs are faulty

• Get a professional diagnosis: Have an auto repair expert check the glow plugs and the rest of the engine to pinpoint the exact problem. 
• Avoid long trips: If you must drive, do so cautiously and avoid highways or high speeds. 
• Go directly to the mechanic: If you can, drive straight to a repair shop. Avoid putting unnecessary strain on the engine. 

Does a flashing light always mean bad glow plugs?

A flashing glow plug light with poor starting and smokey exhaust indicates possible ECU faults or wiring issues beyond faulty plugs. Check the glow plug relay and control module for proper operation. Inspect wiring harnesses for corrosion or damage causing intermittent signals.