What would cause my 2004 Chevy Malibu check engine light to come on?

There are many reasons You might see the check engine light (CEL) illuminate on Your 2004Chevrolet Malibu. I'll run through a few of them, but the best thing to do is to actually retrieve the diagnostic trouble code (DTC) that Your vehicle's computer has thrown. You can do this with a scanner, or take Your Malibu into an AutoZone, where they'll normally read the code and clear it for You, free. Probably one of the most common reasons, and a non-intuitive one, is a loose or worn gas cap. There's actually a fuel system sensor that detects gas vapors, so if the cap's loose, it'll pick up on those. The solution here, of course, is to tighten it, or replace it, if ncessary. The next one's going to be a faulty oxygen sensor. For some reason, these seem to fail often, followed by the mass air flow (MAF) sensor. Both of these are related to engine and fuel efficiency, so You'll want to sort them out as soon as You can. More potential causes are faulty ignition coils, leaky vacuum hoses on the evap system, a faulty catalytic converter and bad spark plugs. Of course, there are many more potential causes, but those are going to be what You'll see a lot of them --- trying to diagnosis this in the blind, though, is silly: Get the code from the computer so You know what's actually failing!