Pathfinder Service Engine Soon Light on Intermittently on 2001 Nissan Pathfinder

2001 Pathfinder... we have been towing trailer (around 4400 lbs/ within mfg limits). It seems that when we are towing, and then Fill up the gas tank, the Service Engine Soon light goes on shortly afterwards. Then after some time of driving (and possibly no longer towing) the light goes off. One time we had it at the repair shop and they said there was no code to read and the light was off when we picked it up. Any ideas? Thank you!

Asked by for the 2001 Nissan Pathfinder