A check engine light most definitely can tell you that you are about to have a breakdown. It's original and most common purpose is emissions control, but it has

A check engine light most definitely can tell you that you are about to have a breakdown. It's original and most common purpose is emissions control, but it has been extended. An overheating engine will cause the check engine light to come on (happened to me last week in an 88 4Runner) and this has nothing directly to do with emissions. Blow a head gasket, and there could be a long list of repairs besides a new gasket. And even if it is a problem that primarily affects emissions, it might still have a bigger problem. A bad O2 sensor can cause your engine to enter open loop and run rich, meaning that you're blowing gasoline out your tail pipe, costing you money. A misfire can cause the light to come on (bad spark plugs, compression leak, bad fuel injector, bad timing, bad lots of things) and can cause reduced power (which might not be noticeable in city driving), burnt valves, and again blowing gasoline out your tail pipe. Blowing gasoline out your tailpipe will ruin your catalytic converter, which you might not care about right now, but will matter if you try to smog your vehicle to sell it. I've had the check engine come on in a number of vehicles I've worked on, and it has always been a more serious problem than a loose gas cap. The previous owner of the above mentioned 4Runner ignored her check engine light. By the time I got to it, the engine, 02 sensor, and catalytic converter were destroyed. If your check engine light is on, scan the codes and make sure that it isn't anything serious.