Glow Plug Light: What It Means and What to Do

The Glow Plug Light shows that a diesel engine's glow plugs are preheating the cylinders, and if it flashes or stays on while driving it signals a fault.

Caution
Can I keep driving? If the light turns off after preheating and the engine starts normally, there is no problem. If it stays on or flashes while driving, the vehicle can usually be driven carefully, but starting may be hard in cold weather and the engine may be in a reduced-power mode, so have it checked soon.
What it looks like
A horizontal coiled wire that looks like a pigtail or spring.
Color
Amber/yellow. It normally lights briefly with the key on and goes out when the engine is ready to crank. Some vehicles flash it to report a fault.

Also called: coil light, diesel preheat light, pigtail light, spring light, wait to start light

Updated 2026-11-07 · By CarWhere Research

Why the glow plug light comes on

Diesel engines ignite fuel by compression heat, so in cold conditions glow plugs heat the combustion chambers before cranking. The light tells you to wait until preheating is done. On many vehicles the same light also doubles as a general engine-fault indicator for diesel-specific systems, so it can flash or stay lit when a sensor, glow plug circuit, or emissions component fails.

Steady vs flashing

Lighting for a few seconds at key-on and then going out is normal. Staying on after startup or flashing while driving means a fault has been stored, often in the glow plug circuit or another engine-management system.

Common causes

  1. Normal preheat cycle at startup
  2. Failed glow plug
  3. Faulty glow plug control module or relay
  4. Wiring or connector fault in the glow plug circuit
  5. Fuel system or sensor faults reported through the same light on some diesels
  6. Diesel particulate filter or EGR faults on vehicles that share the indicator

What to do when the glow plug light comes on

  1. At startup, wait for the light to go out before cranking the engine.
  2. If the light stays on or flashes while driving, reduce load and note any loss of power.
  3. Have the engine codes read to identify whether the fault is in the glow plug circuit or another system.
  4. Test individual glow plugs and the control module if a glow plug code is present.
  5. In cold weather, use a block heater if equipped until the repair is made.
  6. Clear codes only after repairs and confirm the light behaves normally at the next start.

Symbols and colors vary by manufacturer; your owner’s manual has the exact meaning for your vehicle.

Related OBD-II codes

P0380

P0670

P0671

P0381

Glow Plug Light FAQ

What does the glow plug light mean?

The Glow Plug Light shows that a diesel engine's glow plugs are preheating the cylinders, and if it flashes or stays on while driving it signals a fault.

Can I keep driving with the glow plug light on?

If the light turns off after preheating and the engine starts normally, there is no problem. If it stays on or flashes while driving, the vehicle can usually be driven carefully, but starting may be hard in cold weather and the engine may be in a reduced-power mode, so have it checked soon.

How long should the glow plug light stay on?

Typically a few seconds, longer in very cold weather. Start the engine once it goes out.

Why is my glow plug light flashing while I drive?

On many diesels a flashing coil light indicates an engine-management fault, not just a glow plug issue. Have the codes read to find the system involved.

Can a bad glow plug stop the engine from starting?

In warm weather a diesel may still start with one weak plug, but in cold weather failed glow plugs can cause hard starting, rough running, and white smoke.

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Sources

General information written by CarWhere Research, not a diagnosis of your vehicle. When in doubt, stop in a safe place and follow your owner’s manual.