SES light on fill-up

[working on it]

Three times now, when my wife fills up her 98 GMC 1500 5.7, the SES light comes on and stays on.Twice, I had the code checked and reset.It showed that the fuel tank was losing pressure? so I assumed that when the cap was replaced after fill-up, the system didn't achieve the proper pressure.I replaced the cap with a new one, and no code the first time, but again on the next fill-up.What gives?

[Wingnut]

Could be a problem with the filler neck.

[95Sierra2500]

It could be a problem with the filler neck, but are you sure she's tightening the cap properly? It's good advice to turn it past at least 3 clicks. It could be as simple as she's not putting it on all the way.

[Butch 02 Denali]

Could she be leaving the truck running when filling?

[98GMC_20]

My light will come on just from the brand of fuel I put in, that prob has nothing to do with your problem. I live in Alberta "oil country" and we make a lot of fuel here in our province but I swear we sell the good stuff and keep the crap for us albertans. My problem when the light comes on tells me its a rear exhaust manifold O2 sensor but goes away after a few stop starts.