Car wouldn't start this morning?

This morning my 04 Chevrolet colorado I5 would not start. It was turning over
but not actually starting. It has done this before, usually during the winter. This
morning was not cold (73 degrees) but it was very humid (99%). Got a ride to
school and when I got back home it started right up. Took the truck to advance
auto parts to have them read a CEL and the code was P0128 which seems to
be corresponding to the coolant thermometer, not something that I feel would
affect a cold engine from starting. Some other info that may or may not be
related: idles rough occasionally, sometimes at night when coming to a stop
headlights dim and then brighten back up once stopped completely, exhaust
leak right before catalytic converter.