A tractor trailer at the side of the road ideally means that the driver has pulled over for a well-earned nap. Of course it could also mean a breakdown. One troubling scenario to have is having the DEF light come on.

The DEF (diesel exhaust fluid) light is a driver warning system that lets you know when your DEF tank is nearly empty. This affects truck drivers more than it does drivers of passenger vehicles. DEF is, essentially, a mixture that is added to the vehicle's engine to reduce environmental harm by combining with the diesel gas. The DEF light glows when it is time to add fluid, and as to whether it is safe to drive with the light on, yes, it is. But you shouldn’t. If you do, you could be in trouble.

Here’s are some things to know about driving with a DEF light on:

  • Before your DEF tank ends up empty, you will see an alert on your dash in the form of the DEF light. If your DEF drops below 2.5%, the light will be a solid amber. If you decide to ignore it, at the point where you run out of DEF, the light will glow solid red.

  • It gets worse. If you ignore the solid red, your vehicle speed will be reduced to a snail’s pace – 5 miles per hour – until you top up the DEF tank.

  • The DEF warning light can also indicate contaminated fuel. The effect will be the same. This type of contamination most often occurs when someone accidentally puts diesel fuel in the DEF tank.

Most often, loss of DEF fluid is due to driver error. Drivers sometimes forget to check the DEF fluid when they check their fuel levels. This not only results in a loss of power, it can also damage the DEF system itself. The repairs can be very costly, and of course can result in unwanted downtime for the driver.

The solution, obviously, is proactive maintenance. Drivers need to be vigilant when it comes to DEF, so that they don’t lose time, damage their vehicles, and end up in big trouble with their employer. Ignoring the DEF light is never a good idea, so if it comes on, the driver should pull over and have their DEF refilled immediately.

This article originally appeared on YourMechanic.com as Is It Safe to Drive With the DEF Light On? and was authored by Valerie Johnston.


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