Why Is My Air Conditioner Not Turning On Despite Thermostat Set to Cool?

An air conditioner not turning on with the thermostat set to Cool usually means a control, safety, or component fault is preventing startup. The most common culprit is a thermostat or control issue that isn’t sending a call for cooling. It can also be a protective shutdown or a failed part keeping either the indoor or outdoor unit from starting. On older systems, we often find a small safety switch has tripped long before any major part fails. Our technicians handle these kinds of problems every day across the Oak Park area’s older housing stock. If the system smokes, sparks, or trips repeatedly, treat it as an emergency; we offer 24/7 emergency heating and cooling services in Chicago.

A professional diagnosis typically includes:

• Clearing and flushing the condensate drain and testing the overflow/float switch
• Verifying refrigerant pressures and checking for leaks
• Testing safety and operational controls, including high/low pressure switches
• Inspecting wiring connections and motors, with voltage/amperage checks

Turn the system off at the thermostat and breaker if you smell burning or see water around the unit, then schedule service.

Updated: 2026-06-20

Need help in Oak Park, IL? Call (847) 440-5642 or book online. American Vintage did a great job on a repair to my HVAC unit. — Jeff L.