Just was wandering if anyone has had the engine light come on and the display say the check gas cap. Can anyone tell me why this is hapining thanks.This is on a 99 deville.

 

OnmostGM vehicles from 96 and up are equiped with obd II. this diagnoses how the car is running as you probably already know this. Why your car may say "check gas cap", is because youpossibly may not have you gas cap on very good , or the evap canister hoses may have a leak, or you may have lost the gas cap (lol). because what happens is while you drive, the gas in the tank sloshes around and creates pressure, like pop in a pop bottle. so the obd II senses this and regulates the pressure by sending some of the fuel vapors into the fuel evap canister which can be sent into the engine to be used instead of wasting it. If your gas cap is off there is little or no pressure so thats where OBDII lets the driver know that you may be losing fuel somewhere. because it cant sense any pressure. This is common on many vehicles whith this system. my 2000 Grand am does this to me every time cause i dont put the cap on very tight or dont put it on straight. Or i lost it which did happen.
~hope this helps and good luck.~
also if you want to make a quick buck off your friends losen their gas cap and let them think something is wrong with their car and say "wel i can fix that for $20 bucks " then let them give you the car and go around the block tighten that cap turn the car back on and then wait awhile and thell think you fixed something serious. I did it to a friend but i never charged him cuz he was so mad when he thought it was something else and all i did was tighten his gas cap .