DEF Light Comes On? Here’s What You Should Do

Tim Miller
DEF Light


The diesel exhaust fluid (DEF) level is low or the system malfunctions.

DEF Light


  • Low Diesel Exhaust Fluid Level
  • Contaminated Diesel Exhaust Fluid
  • Clogged Injector
  • Malfunctioning Sensors



Easy to fix


What Does It Mean When the DEF Light Comes on?

diesel exhaust fluid symbol

Diesel Exhaust Fluid (DEF) or AdBlue is a fluid made up of urea and deionized water that is used to reduce the emissions of harmful nitrogen oxides (NOx) from diesel engines.

When the DEF light comes on, it indicates a low DEF level, contaminated DEF, or malfunction in the system.

If the problem is not fixed, the light will continue to flash. Your car will typically go into limp mode, which reduces engine power and speed.

Sometimes, the check engine light will also appear with the DEF light. You should diagnose and fix the problem as soon as possible to avoid damaging your vehicle’s emission system.

Can You Drive With the DEF Light?

Yes, you can drive as long as there are no other warning lights or signs of engine problems. However, you should take immediate action to refill the DEF tank as soon as possible. Ignoring the DEF warning light can lead to serious problems with the vehicle’s emissions system and may even cause your vehicle to break down.

So how far can you drive with it?

Most diesel vehicles will provide a range of miles that you can travel before the DEF tank is completely empty, so you should use this information to plan your refilling accordingly.

Why Is My DEF Light On?

1. Malfunctioning Glow Plug
malfunctioning glow plug

A faulty glow plug is the most common cause of the glow plug light illuminating in your diesel vehicle. When the ECU senses that one or more glow plugs are not working properly, it will turn on the glow plug light to alert the driver of the issue.

There are various reasons why glow plugs can malfunction, including:

  • Age and wear: As glow plugs age, they can become less effective at heating up, which may eventually cause them to fail.
  • Overheating: If the engine overheats, the glow plugs can be damaged and fail.
  • Contamination: When dirt or debris accumulates in the combustion chamber, it can build up on the glow plugs and cause them to stop working correctly.
2. Malfunctioning Sensor
The sensor in the control module is responsible for monitoring engine temperature and sending signals to the control module to activate the glow plugs if necessary.

If the sensor malfunctions, it may send incorrect signals to the control module, turning the glow plugs on unnecessarily. This can cause the glow plug light to come on.

3. Faulty Glow Plug Relay
The glow plug relay is responsible for supplying power to the glow plugs.

When the glow plug relay is not functioning properly, it may not supply enough power to the glow plugs, which can cause them to fail or operate at reduced efficiency. This can cause the glow plug light to come on.

4. Electrical Issues
Over time, wires or electrical connectors become loose or damaged. This affects the operation of glow plugs and triggers the light to come on.

What to Do If the DEF Light Comes on

Step 1: Pull over and visually inspect the DEF system

When the DEF light comes on while you are driving, it is best to find a place to safely pull over and check the DEF system.

  • The DEF tank is low; refill it with the appropriate type of DEF.
  • The DEF has signs of contamination; flush the tank and refill it with fresh DEF.
  • The DEF tank has been damaged; have it replaced by a qualified mechanic.

If the DEF light still stays on, move to the next step.

Step 2: Use an OBD2 scanner to identify the problem

As I mentioned above, numerous factors can activate the warning light. When a malfunction is detected, details about the malfunctioning components are stored in diagnostic trouble codes. That’s why we need a scan tool to read the codes. From it, you can get valuable information about the underlying cause of the DEF light turning on, such as a malfunctioning sensor or a faulty DEF injector.

If you would like to learn how to use an OBD2 scanner, follow these steps:

  • Connect the scanner to the car’s OBD port.
  • Turn the ignition on.
  • Select the “Read Codes” function on the scanner to search for the codes.
  • Figure out what the code means by using the DTC Lookup Library on the scanner or going to the OBD2 Codes Lookup to find meaning, causes, symptoms, and how to fix it.

Step 3: Repair malfunctioning components

Once you find the problem, make repairs immediately. If you cannot handle it yourself, consult a mechanic.

Step 4: Reset the DEF light

Once you have completed the above steps, it’s time to reset the DEF light.

You can clear the trouble codes by selecting the “Clear” function on the scanner. 

Thus, drive for 50-100 miles. The DEF light will disappear.

What’s Next?

Now that you know how to handle the DEF light in your vehicle. In case you encounter different situations, share your story in the comment box below.

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Information sourced from the owner's manual.