You may not have enough start cycles to clear the code. Have you cleared the code with the reader? If you have and it comes back it's probably a cracked hose or fitting below the filler cap like one of the various hoses or connectors around the gas tank.
I had a Dodge Dakota that did that. After several gas caps I took them all to the shop. the checked it and they smoked it and could not fix it. No leaks.
What I did to stop the problem...Took the original Cap and tightened it but stopped before it clicked. Problem solved. Not a problem for two years.
Just tighten it and stop before it clicks. If it clicks then remove the cap and re-tighten it with a little less pressure.
Here's a DIY smoke machine. It's a HF hand transfer pump for <$10.
Attach it to the intake as shown. Stuff a rag in the exhaust pipe and stopper the throttle body intake.
Light the cigar and pump smoke in and watch for it to show smoke somewhere.