P0128 Light Jeep Wrangler

What causes the p0128 light to come on? I just got
a 2013 Jeep Wrangler unlimited sport yesterday.  
As soon as I got it, I hadn’t to get gas because
they left it on e, I then drove to work and when I
was driving home from work at the end of the night
the check engine light popped on halfway through
the drive. This morning I turned the Jeep on and
the temp gauge was all the way on C. I turned the
hot air on and it was definitely blowing hot air, I let
it sit for a few minutes then it finally got to the
halfway mark and stayed there my entire 25 minute
drive. I get to work and I check the code and it
shows no code now, just the empty dashes. Did I
actually fix the problem or does the code only
show itself a few times? I’m just wondering
because I have to get emissions tested soon and
driving with a check engine light makes me
nervous!