I have a Lennox Furnace. It was turning on but there was no heat. I just had a service man here who took out the heat sensor and cleaned it. He put it back in and the heat worked. He did not have a new sensor with him but he recommended that I replace the one he just cleaned. Should I? The heat is working fine. If the sensor is cleaned and working why would I need a new one? The service call cost $194, and a new sensor is in the $165 range.

Don't worry about it. Flame sensers can be cleaned many, many times before they need replacing. Just an excuse for them to sell you a new flame senser. If you watched him clean it, you should be able to clean it the next time. Simply pull it out, clean it with steel wool, fine sand paper, or emery cloth. Really no need to replace the flame senser if the furnace is working fine.