What else could it be?

My CE light came on a while ago and
the codes were knock sensor and a
coolant sensor, so i replaced the
coolant sensor (it was cheapest) as
well as thermostat. Now when i turn
my car on the gauge goes halfway
immediately and climbs from there.
Also i smell some kind of coolant
burning but i see no smoke and there
is antifreeze. Nothing is leaking that i
can see underneath. The gauge used
to stay JUST below halfway. And
when im driving the gauge goes to H
but slowly goes down between H
and the three quarters mark if im
doing 65-70mph.