I have a Lennox AC13 condensing unit that comes on and then cuts off. It will not stay on

Check the fan proof switch... the thermostate turns on the fan if after a few seconds, about 90 is the fan proof switch does not activate the controller thinks the fan is not running the shut the unit off... the fan switch has a tube connected to it... this tube is also connected to the out put of the fan... the moving air from the fan goes through the tube and pushes on a diaphram in the proof switch which in turn closed a switch and sends a signal the controller the indicated the fan is running... if the proof switch is bad or the tube is blocked... then you get no proof of air flow..