There are hundreds of things that can couse a check engine light. You need to have it checked for codes.
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.
I have a daewoo matiz that has low power when the engine light comes on I have changed the spark plugs/leads/distributer cap/distributer/and coil checked the timing changed the fuel filter air filter and it still looses power when the engine light comes on. You have spent a lot of money and you still have the problem. Do you not think it might be time to take it to an expert and have it repaired correctly. You would've been money ahead right at this moment.
Have vehicle scanned to determine problem and clear codes
You need to have the computer codes read at the dealer or a parts store ( lots of them will do it for free ) the issue that your truck is having may have nothing to do with the gas cap. If you did have the codes read and a gas cap was the answer then disconnect the battery or better yet remove the ecc fuse for about a minute to reset the computer and see if the problem returns.
Any vapor leak from the fuel system can cause a "gas cap" light.
Check your gas cap. Make sure the seal is still good. This will cause the check engine light to stay on and is an easy fix.
You replaced the cap and it will take quite a few driving cycles for the light to go off if that was the culprit. Try disconnecting the negative battery cable for 5 minutes and that might reset the light.
it is a hyndai that's why it is a hyndai that's why
A few of the top reasons a check engine light are on is the gas cap is not tight, the oil may need changed, or a tune up is needed. A mechanic can run a diagnostics test for a small fee.
I changed erg valve o2 sensor and gas cap but lights back on what do I do
Could be about 100 different things but first thing to check would be your gas cap If it is loose or not sealing it will turn on the light. Note If that is the problem after it's tightened light will still stay on for 2-3 days have you changed the air filter. I have a 99, the light came on because the air filter needed to be replaced.
I had my 200 blazer scanned an was told it was a bad fuel tank cap. I replaced it and the light is still on.