The Glow Plug Warning Light is ON: Causes & Solutions

A little orange coil symbol appears on the car dashboard while driving. What is it, and what’s the matter? It’s the glow plug warning light, and it’s not a good sign for your car.  So what are the reasons behind this light indicator? Let’s deep down to find the solution: 

What Is A Glow Plug On A Car? 

First, let’s go over what does a glow plug do? And does every vehicle have this little part? A glow plug is an ignition control device only used in diesel engines to achieve cylinder temperature faster to ignite the fuel, especially in cold weather.  

Diesel engines do not use spark plugs to ignite fuel like gasoline engines, but fuel is injected, and compressed under high pressure and self-ignites. In cold weather, the engine temperature is low, and the engine speed when starting is low (about 100 rpm), increasing heat loss during the compression stroke of the engine, so the fuel injected into the combustion chamber is very difficult to self-ignite, that’s why the diesel engines need glow plugs. 

The glow plug is located in each cylinder head of your car so when the diesel car starts, this part heats up rapidly and increases the temperature of the combustion chamber to a point where the diesel fuel can ignite easily.

The Glow Plug Warning Light Is On: Why?

When you notice the glow plug indicator light on the car dashboard, it’s mostly due to a bad glow plug or there are some problems with other car parts we will explain to you below. Now, let’s into the potential culprits behind that stubbornly lit glow plug light:

1. Faulty glow plug 

When the glow plug in your car starts to fail, the ECM recognizes and sends out a warning light on the dashboard to warn the driver. At this time, you will see a spring-shaped light glowing after the car starts up. Some cars go to limp mode till the issue is fixed. The glow plug can degrade over time, triggering inadequate heating and an illuminated indicator.  

2. Glow plug control module issues

Sometimes, it could be a glitch with the control module. Do you wonder how the overall operation of the glow plug is monitored and controlled? The glow plug control module does it. This part manages the timing and duration of the glow plug operation. If this module is malfunctioning, it will not receive the right signal to turn off the glow plug light, so at that time, you’ll watch out for the glow plug light on your car dashboard and also quickly find it difficult to start the engine. 

3. A bad temperature sensor 

A faulty temperature sensor can cause the glow plug warning light. Modern engines rely on temperature sensors to regulate the glow plug operation. Some diesel engines have sensors that monitor the temperature of the combustion chamber. A faulty sensor can provide incorrect readings to the ECU, so the ECU will decide the glow plug light to stay on even when the engine is warm. 

4. Wiring issues 

Another reason could be with the connections or wires that power and connect the glow plug to the ECU. Damaged wiring or poor connections can disrupt the electrical circuit between the glow plug and the control unit, resulting in the persistent light indicator. 

Additionally, a short circuit can overload the circuit, blow fuses, or even damage the glow plug relay or ECU. The warning light will likely come on, and you might experience other electrical problems.

5. Glow plug relay failure

The glow plug relay failure is also one of the common reasons. The cause may be a poor contact control circuit at the starter switch, the relay coil connectors, or related to the heater timing controller. If the wire is broken internally, it can also cause this condition. The glow plug relay acts as a switch, sending power to the glow plugs when needed. If the relay fails, the glow plugs won’t receive power, and the warning light will illuminate.

>> Related post: Do Diesel Engines Have Spark Plugs?

Solutions For Flashing Glow Plug Warning Light

Now, you know what might be causing the issue. So how to fix the glow plug staying on or the glow plugs light flashing while driving. Solutions for the glow plug warning light usually involve checking a few parts and doing some repairs. Here are our solutions: 

Check the glow plug

Let’s start the first step by looking at the glow plug to check its condition. Visually inspect the glow plugs for any sign of wearing or damage such as cracks or burns. Replacing any worn-out or bad glow plug If possible. Though the glow plugs can last for as long as 100000 miles, they will gradually deteriorate due to wear and tear over this period, so if you notice one of the symptoms below, you should check and replace your glow plug ASAP:

• Hard starting
• Engine misfiring
• Rough idling 
• White or black smoking 
• Decreased fuel efficiency

The good news is that not only are glow plugs incredibly long-lasting, but they’re also amongst the most cost-effective car parts to replace. So it will not cost too much for you to replace a glow plug.

Inspect the wiring system

If a new glow plug doesn’t remedy the problem, let’s take a look at other parts. Carefully examine the wiring harness connecting the glow plugs to the ECU for any signs of damage, corrosion, or loose connections. If they are, they will need to be cleaned or replaced. 

Scan for error codes

Sometimes you will need to use special tools such as an OBD-II scanner to figure out if there are any problems with the control module. You can retrieve diagnostic trouble codes from the ECU. These codes can provide valuable information about the specific problem with the glow plug system. Additionally, you can test the glow plug control module according to the manufacturer’s guidelines or seek professional help. 

Temperature sensor testing

Test the temperature sensor’s resistance with a multimeter to ensure accurate readings, replacing the new one if necessary. 

>> Read more: Bad Glow Plug Symptoms Every Driver Must Know

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Wrapping up 

Understanding what this light means and how to deal with it is key to keeping your diesel car running smoothly. It’s a good idea to have a mechanic take a look and diagnose the glow plug system. In case, you need to replace a new glow plug, Automotive experts recommend that drivers should not choose low-quality, cheap glow plugs because when used, they cause serious troubles such as the glow plug exploding, the glow plug breaking, and remaining in the engine, falling into the combustion chamber, which is very dangerous.