Kitchenaid built-in refrig 15 yrs old cold...

Question

freezer at -8 degrees but refrig section holds at 42to 44 degrees. just had compressor,evaporator and heat exchanger (SEALED SYSTEM)replaced along with pan heater on dec 19,2008. could the thermostat be bad? the damper in the refrig has cold air blowing in from the freezer and does not seem to be stuck. can open fully when control in refrig is put to ''coldest'' . does the thermostat regulate how much cold air gets into the refrig? I made sure that nothing is blocking the damper and the cold air return is not blocked or clogged