Malfunction Indicator Light
Any number of reasons... typically it's an error that is detected by the truck's computer system (ECU). These errors can be read by an OBDII code reader (they're pretty cheap on Ebay). Good Luck.
What can I do about it.
Nothing unless you get a code reader, and repair the problem.
There are a couple things you can try before taking it in. First, a common cause is the gas cap not on correctly. Take it all the way off and retighten. If this fixed it, the light will go off after a few miles and maybe turning it off and on again. That happened to me and it fixed it. Abother time It was after service. About 400 miles afterwards it came on. In this case I traced it down to finding out They didn't put the air cleaner cover one tight. There are clasps on one end and then it slides off the other. Towards the rear they failed to slide it over the tab correctly. I went to change the filter and found it. Drove it....turned it off and on again and the light cleared.
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