When the Units come in they are thoroughly cleaned, disassembled and inspected. This requires cutting apart the numerous rivits without destroying the fragile plastic case. Next the pieces are carefully pulled out of the black mastic glue taking care not to bend, break or crack any of the parts. The heater core is replaced with a brand new heavy duty unit (much better then the original) and all of the componants are reassembled and tested from the vent dampers to the blower motor.