Arrived onsite due to customer complaint of no heat inside their home. Began our furnace diagnostic by reading the flashing code on the control panel which gave us 3 short blinks followed by 3 long blinks (Status Code 33). Went through the system safety’s and all of them tested out. We found the flame rollout switch to be tripped so we reset it. Set call for heat on the thermostat and the unit kicked on. Unit was putting out 100 degree air. CO measurements in the exhaust flu rapidly increased over 1500ppm until combustion analyzer reached over maximum capacity. These measurements and then tripped the high limit indicating likely plugging of the secondary heat exchanger which is common to this model. While some manufacturers warranty may still exist it would have to be provided by a carrier dealer. Given the age of the equipment we recommend a replacement. Pricing for a replacement was included. Based on the measurements we took the unit is unsafe to use. We have disconnected the gas