Check engine light on. I checked gas cap and it was not tight. Tightened it and light still on

The "gas cap light" actually does not mean the gas cap is loose or defective.It is telling you there is a leak in the fuel system somewhere,usually a vapor line or stuck or defective valve.Not very often is it the gas cap that causes the light to come on.Time for a scan and a smoke test to fine the leak.

Go to an auto parts store and have them read the code(s) to tell you what the problem is. If there are no other problems, it is safe to drive there.

Likely a problem related to the pollution control equipment - catalytic converter, PCV valve, etc. Hard to tell without the codes.

Is your gas cap on tight? Often that is the problem. It may take a couple of drives and/or days to clear the Check Engine light after you have tightened the gas cap.

Good luck !!

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There are dozens of reasons why a "Check Engine" light could come on. Until you get the codes read, you can't begin to have a reason why. Go to your local auto parts place, most will read the codes for free.
Infinitis do have a trick, though. If the last time you filled the tank, the gas cap didn't get fully tightened, a day or so later, the Check Engine light comes on. Confirm the cap is tight, and if it wasn't, the light will go off tomorrow.

The gas cap light is a generic light that indicates an emission problem. Usually a vacuum/pressure leak within the system. The gas cap is usually the first thing to check, however may not be the problem. There are numerous sensors and lines that control the vacuum/pressure/ of the fuel vapors and one of these is loose/disconnected/defective causing this light to come on. I would recommend inspecting hoses under the hood for any that may be loose/disconnected. You may also require a scan test to view sensor data to isolate the source of the light

If the code would have been due to the cap problem then the code will go away. Try resetting the code. Also try diagnosing again after disconnecting the battery and then reconnecting it after a few minutes.

For a 2000 Honda Accord LX, the check engine code P1456 simply means that your gas cap is off or isn't tight enough. It wont go off immediately. It make take up to a weak for the check engine light to turn off once the gas cap is tightened enough.-------------------------------------------------------------------------------------
Renault CAN Clip

99% of the time, the gas cap wasn't tightened properly, or is leaking. Make sure you twist it tight and it clicks a few times. If the "Check Engine" light stays on after a few days, buy a new gas cap. Again it may take a few days to clear.

IF IT COME ON EVERY TIME YOU GET GAS.GAS CAP HAS BAD SEAL IT MORE LIKELY YOUR PROBLEM BUT LET AUTO ZONE SCAN IT FREE TO SEE WHAT TROUBLE CODE YOU ARE GETTING.

Your check engine light usually means there is an issue with your emissions control system. The last time mine came on, I found that my gas cap wasn't on tight enough. I turned it on tightly and the light went off after 30 miles or so. Start there.
If you've tightened the cap and the light is still on after 30 miles, you may have a bad gas cap OR maybe you just got some bad gas at your last fill-up. Try a few gallons of high-test and drive it a while. If nothing changes, a new gas cap is probably the next cheapest option. Good luck.

change the fuel cap