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Home Blog What Does The 'Diesel Exhaust Fluid Low' (DEF) Indicator Mean?

What Does The 'Diesel Exhaust Fluid Low' (DEF) Indicator Mean?

If you have a GM truck that runs on diesel, you may have noticed a DEF (diesel exhaust fluid) indicator pop up on your dash. If your dash has an LED screen, you may have seen a message that reads something like:

DEF warning lights
  • "Service DEF system"
  • "Diesel exhaust fluid level low"
  • "Exhaust fluid quality poor"
  • "Exhaust fluid pressure low"
  • "Service DEF system"

Some trucks come with an indicator that tells you how much DEF you have left in the tank. It's an important indicator in your truck, and you should not ignore it under any circumstances.

What The DEF Indicator Tells You

The DEF indicator in your truck exists to inform you of several different important issues:

  • The diesel exhaust fluid level getting dangerously low
  • A pressure problem in the DEF system
  • Contamination in the DEF tank (for example, when someone accidentally put diesel fuel into the DEF tank)

All these issues can get very serious if not addressed in a timely manner. That's why GMC designed its diesel trucks to show the DEF indicator light as soon as an issue crops up.

Why The DEF Indicator Is Important

DEF pump

When there's an issue in your truck's DEF system, it can become serious quite fast.

If your truck runs out of DEF, if the there's a pressure problem within the system, or if the DEF is contaminated, the system will enter "degraded" mode. That means your truck is limited to a very low speed, like 30 mph. Your truck won't go back to normal until you refill the system with DEF.

If there's something wrong with the DEF system, the DEF can no longer reduce NOx emissions. That means your truck is creating more emissions than the amount permitted by the U.S. Environmental Protection Agency. The agency states that trucks can't run without a functioning DEF system.

That's why your truck has a DEF indicator. It will alert you as soon as something goes wrong within the DEF system. You'll have enough time to fix the issue before your truck's DEF system enters "degraded" mode.

Is It Safe To Drive With The DEF Indicator On?

When the light first comes on, it may be safe to keep driving the truck. The longer you drive your truck without addressing the DEF system issue, the less safe it gets. You should pull over and refill the DEF fluid as soon as the DEF indicator comes on. You can get some DEF fluid at any GM dealership as well as most auto parts stores.

If the DEF indicator stays on after you refill the fluid, there may be a problem with the DEF system. That, or there's contamination in the DEF tank.

Some trucks come with a color coded DEF indicator. When it first comes on, the color may be amber. If the issue gets bad enough (for example, if the DEF level drops below 2.5%), the light turns red.

The bottom line: address the DEF indicator as soon as it comes on!