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Hi, did he bypass the high limit to see if it was bad? Sounds like you are getting false codes and you have a problem in your module or main board, which ever you have. The way it is acting, saying the high limit is open but the tech says its not, I would change the board. Shastalaker7
This sounds like it may be a control board issue. If the thermostat has been changed, and the unit operates correctly after it has been reset, then I am thinking the circuit board in the unit is faulty. When the unit stops working, check for a blinking or steady red light on the circuit board. Alot of the time this will be on or blinking and there will be a legend that tells you what each light stands for. This can help you troubleshoot the issue.
The manual selection on the thermostat bypasses the fan relay. When in Auto, the fan relay turns on the blower. You are probably looking at replacing the circuit board. As far as the three blinks, on the reverse side of the front panel there is a wiring diagram and error code chart. It will give you a starting point where to look for a problem. The three blinks could indicate an open limit because the blower is not operating. Keep the blower on manual and let the furnace run normally until you can figure out what the problem is.
The blower may cycle on and off to cool the heat exchanger. The fan limit still senses the heat after the furnace shuts off and the limit goes up the same as when the furnace first starts. You could adjust the settings on the fan limit.
Your furnace is likely getting too hot check your filter. make sure the fan is running. if you have weak air flow you may have a blocked heat transfer coil. you should have your heating and cooling system serviced yearly by a qualified company to inspect and test equipment.