Chevrolet: 2004 Impala Heater

I know this is a Malabu site but hope you may help.

The dual heater only gets warm on the pass side. Driver side has no heat at
all. The pass side gets warm but not hot like it should.

fan, defog, vent and floor controls works fine

Engine temp is good at 190

thoughts?

got to be one of these and dont want to shotgun stuff to figure this out.
are any of these more common to fail?

Heater valve
Damper blend dorrs
AC/Heater control

thanks