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DIY: Why is My Check Engine Light On

There is nothing more annoying or potentially troubling than the dreaded check engine light. Is your light on to indicate a real problem, or is it just a false warning? Check out this diagnostic guide to find out what is really going on.

By Tony Greene - June 28, 2018

mmarshall

Step One is correct. Loose-fitting gas caps, either from excessive wear over time, or just not tightening them enough clicks, are one of the most common (and cheapest) reasons the light is triggered. The EPA requires a tight seal around the filler-pipe, so that raw gas fumes cannot escape into the air. If the seal is not tight enough, the sensor will detect it and send a signal to the engine computer, and the light will flash. Sometimes, simply another click or two on the tightening will do it...other times, a new cap might be needed. It could also mean that you left your gas cap back at the station, sitting on top of one of the pumps......yes, a lot of people mindlessly do that, particularly after the cap's tether-cord is broken (which also happens over time).

Of course, some Ford and GM vehicles don't use traditional gas caps....they have the capless filler-pipes that seal themselves with a spring-loaded flap when you pull out the nozzle. In that case, the seal mechanism may have to be repaired or replaced.

It should also be noted that, even with a successful repair or adjustment, the light doesn't always go off immediately. Depending on the programming, it may take a couple of hours of driving or start-stop/engine-off/on cycles for the light to finally reset itself and go off.