My air conditioning unit outside will be running, but the air handler in the garage will not start. I have to turn off the thermostat and turn it back on to get the handler to start. Is there a switch or valve on the handler that can be replaced? It is a Janitrol

Sound like could be loose wire at thermostat or bad stat not calling for fan operation. I would check with G terminal on Tstat which on most units is the fan lead. Make sure good connection. Next try switching Fan control at Tstat to ON. If air handler runs it would seem Tstat isnt telling air handler to run in cooling mode. If that is the case try putting new Tstat on. Brian