2001 GMC Sierra check engine light

Truck seems to be running fine, good fuel efficiency etc, it is due for smog check this month. Recently I helped a neighbor jump start his car, unfortunately I momentarily reversed the cables to my vehicle. I haven't made that mistake in many years. Now I am getting a check engine light that repeats within 60 miles of clearing the error code. The dealer reading the code said it could be a loose gas cap or a sensor problem. Tightened the cap, drove it for thirty miles and it failed to pass smog check because of flags that would not clear, at this time the check engine light had not yet returned. I am wondering if it might be related to my mistake when jump starting my neighbor.