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A technician told me it was the electronic control board that has the blower relays on it , I replaced it and still has the problem, I've been reading here online about a limit switch for the blower but I don't see it in my unit. Where else should I check, no heater and no AC

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  • What is the make and model number of the unit? Can you include a high quality photo of the schematic (should be found printed inside the unit somewhere)?
    – Tester101
    Sep 16, 2015 at 10:27

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The limit switch isn't on the control board; it is an external part that is mounted in the plenum above the burners. Wires run from the control board to the limit switch.

If you also have no heat and no AC, then check that there's 24 volts present on the control board. Some control boards have a normally closed relay that turns off the fan by pulling their switch open after power is applied.

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