I have a thermal zone furnace and have way through the cycle it shuts off and and starts the cycle over. I have replaced the flame/heat sensor. Any other ideas on what this could be. Could it be a ventilation proiblem?

Any improper setting, such as dirty exhaust, clogged air blower fins, a missing screw to the chamber, dirty house air blower fins, improper gas pressure setting, water in the tube that comes from the bottom of the pressure switch that is removed and blown through then replaced, could be the problem. No one single fix for all situations. That's why a good tech will do all the checks during an annual tune-up and give a copy for reference of what they were and should be.

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Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.