Arrived on site to perform furnace diagnostic on 2005 carrier heater. Upon arrival client stated that they moved in two years ago and noticed that the heater blows hot and cold sometimes but it’s operational at this time. Performed full diagnostic on Heater found multiple points of concerns. Measured temperature rise on furnace and found that it had a 65 degree rise. This is an indication that the furnace is overheating past its design temperature rise of 30 to 60. Heat exchanger should be inspected for any defects or cracks Due to constantly overheating over the years. Flame sensor is heavily oxidized and was cleaned off. Indoor blower motor has oil leakage which can possibly lead to a locking up or overheating. Ducting is significantly under sized and return intakes are as well. Discussed all findings with clients and they are aware of all possible failure points within the system. Client reached out to a family friend and he will be returning to do a diagnostic for them as he is i