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Check for 24 volts *between* R and C....tell me what you get.
You should get 19 to 24 volts from R on the thermostat terminal strips either in the furnace or at the thermostat to *ground.
Sometimes C wire will not give a voltage read to ground, in that case the system was designed to have a grounded common (C) wire. So that you need to measure voltage
**between** R and C.