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Glow Plug: What Does a Coil Light Mean?

Jun 4, 2021 ·

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The way diesel engines are designed, they sometimes require help when starting, especially in cold temperatures. They feature glow plugs, which help heat the air and fuel mixture present in the combustion chamber to ensure your car’s engine starts efficiently.

The temperature inside the cylinders is raised by the glow plugs during the process. This helps to pressurize and detonate the diesel fuel. Typically, this requires a driver to wait a short period of time before the engine is fully warmed up and ready to start.

The dashboard on a vehicle is equipped with many symbols to allow drivers to know the status of different parts and systems. This includes the coil light, also known as a glow plug light.

There are a few different reasons why the glow plug light might come on, including cold starts or to indicate malfunctions. Keep on reading to find out more and what it means for you as a driver.

Illuminated Glow Plug Light and Cold Starts

As covered in our gasoline vs. diesel engine post, diesel engines do not use spark plugs. They rely on the heat generated by very high compression to ignite the air/fuel mixture in the cylinders.

When diesel engines are cold, they can be difficult to start. To help overcome this challenge, diesel engines use glow plugs during startup to warm the air in the cylinder, which causes the coil light symbol on the dashboard to illuminate. Once the engine is running, the glow plug no longer plays a role in the combustion process.

The total time needed for a glow plug to warm up varies depending on the vehicle and the ambient temperature. Usually, glow plugs take between five seconds and one minute to sufficiently heat the engine. At that point, the dashboard coil light symbol should turn off, and the driver should be able to start the vehicle. 

In addition to glow plugs, diesel engines may also use other starting aids, including:

  • A coolant heater
  • An ether injection kit
  • An oil pan heater
  • A block heater
  • An air intake heater

What Does a Blinking Glow Plug Symbol Mean?

A blinking glow plug or coil light symbol on the dashboard indicates a malfunction. While there isn’t a quick fix to determine the specific malfunction, a skilled technician should be able to diagnose the problem.

Issues with the glow plug itself can cause the blinking coil light to illuminate, including a loss of heating ability. There could also be an issue with the timer, which is what regulates how long the glow plugs operate. If the timer is off, it can cause the glow plugs to wear out.

Symptoms of faulty glow plugs include a loss of acceleration or a loss of overall power in the engine. You might also notice a drop in fuel economy or even engine misfiring. Be sure and take your car for diagnostic testing if a blinking coil light comes on, as a trained technician should be able to identify and fix the issue.

Can You Drive With the Glow Plug Warning Light On?

If your glow plug light has come on and you’re wondering if you should drive your vehicle, the answer depends on whether the light is blinking or not. If the glow plug light is solid, most cars aren’t able to be fully started until the light has turned off and the engine is fully warmed up.

In the case of a glow plug light flashing, if you need to keep driving, take it easy and avoid high speeds as much as possible. Many times, an engine will go into “safety mode,” and you might notice a drop in performance.

The purpose of a blinking glow plug light is to alert you as a driver to a potential problem, so taking it seriously and getting it checked out when you can is important. This also goes for a glow light that comes on and doesn’t turn off, even once your engine has fully warmed up.

Diagnose a Diesel Glow Plug Indicator Light With Training From UTI

You can learn the ins and outs of diesel systems and diagnose problems after receiving training from Universal Technical Institute. The 45-week Diesel Technology program helps prepare students with foundational knowledge they can apply to a career in the industry.1

The Diesel Engines Service & Repair course covers engine repair and operation. Electrical diagnostics are also covered, giving you the knowledge you need for glow plug inspection and maintenance.

Expand your knowledge and prepare for a career doing something you love. Find out more about the program by requesting more information here.

YOU COULD START YOUR EXCITING NEW CAREER AS A MECHANIC OR TECHNICIAN TODAY.
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1) UTI is an educational institution and cannot guarantee employment or salary.

2) For program outcome information and other disclosures, visit www.uti.edu/disclosures.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.

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