Discovered secondary completely rotted out,I showed customer cracks in secondary heat exchanger explained unit is unsafe to operate,customer decided to replace heat exchanger I strongly recommended to replace furance explained to customer new heat exchanger will crack again just a matter of time tech return for this Thursday between 11- 1 to replace heat exchanger need to collect 1800.00 when completed. Replaced primary and secondary heat exchanger,cycled unit discovered unit going off on code 41 when kick into second stage which is blower motor has ramped up 250 rpm check all wiring connections checked module, customer might have interment problem with circuit board some times it completes the cycle other times it does not cycled furnace multiple times could not get cold to appear again co reading was 0 ppm,