Arrived on site to perform fall tune up on a 2 2005 carrier units. Client stated no issues. They barley used heaters. Cycled both heater and good. Filter were dirty and replaced. Blower assembly and whee on both units were okay. Slight dust as it entered In past years. Both motors are amping in range. Both inducer motors are good. All safety switches are operational. Flame sensor and igniter are both good. Found both units with a high temperature rise of 50 on both. Static pressure was in range on both but a bit high on both units and advised client to findings. Due to high temperature rise and static pressure advised client would like to perform safety inspection of the heat exchanger. Client approved to pull main living room unit. Pulled heat exchanger out and inspect and found heat impingement and 3 chambers compromised due to cracks on chambers stress point. Showed client findings and advised unit is not safe to operate due to findings of over heating. Client is aware and stated h