Service engine soon light

That light means that the computer has recorded an error. That error may or may not have anything to do with the gas cap. You need to get a scanner and find out what the error is, because until you know what the error code is, you have no idea what caused it or what the problem relates to.

There are millions of things that can cause different errors, so I do not know why you are fixated on the gas cap. If the gas cap does not appear damaged and is sealing properly, there is no reason to replace it.

--scott

It seemed odd that the service light goes off when I tighten the cap. If the "service engine soon" light is "off" will a computer scan find the problem? I guess that's my major question.

Depends what the code was.

Depends on what the code is and what kind of scanner you have access to.

--scott

This omits:

1. Modern dashboard indicators (e.g. gas cap open, door open) merely multiply the number of things that will eventually malfunction, sooner or later . . .
2. Interconnection of indicators for "Gas Uncapped" and "Service Engine" may be standard in many brands. They are so in the Ford Focus, where malfunction is common enough to get a mention in the owner's manual, not to mention Ford trouble newsgroups. I have seen this in my Focus 3 or 4 times, ignored it each time, it eventually vanished, and there were no engine problems . . .

Absolutely, which is why a gas cap pressure test is part of an emissions check.

It could either be a broken or shrunkenseal around the threaded part of the cap, or it could be a broken pressure/vacuum relief mechanism inside the cap itself. You couldn't see the latter, and might not see the former if the seal is simply hardened and shrunken a little.

I sure wouldn't go to a dealer$hip for that kind of part, provided that my local auto parts store carries a direct replacement. Don't get some generic part, get one specific to your make and model of car. Unless its a limited-production exotic, which a Mitsubishi isn't, any auto parts store should be able to get the correct cap if they don't already have it on the shelf.

Usually you can tell that a code has been set and then cleared. Others, such as various sensors out of range, can't be cleared except by a code scan tool. The tool will also show you "impending" codes, which means that the engine computer has detected an error, but it hasn't detected it enough times to set the CEL. The computer does have logic in it to throw away the occasional erroneous reading.

LOL. I was wondering why my light went on yesterday after filling up the gas a few days earlier. It hit me this morning, I went for the CA smog check yesterday and was driving back home when the light came on. The tech would have checked to see if I had modified the line I insert the gas nozzle in. I tightened the gas cap again and the light went out.

I'll get the part today. If that solves the problem I'll feel good. If not, I'll get the car checked out.