Can a relay cause AC not to work?
Yes. A faulty relay can prevent an air conditioner from starting by failing to energize the compressor or blower. Relays act as switches in the HVAC control circuit, turning high-power components on and off in response to a thermostat or control board.
In HVAC systems, relays are essential interfaces between low-voltage signals (typically 24 volts from the thermostat) and high-voltage equipment (the outdoor condenser and indoor blower). When a relay fails—whether the coil doesn’t pull in, the contacts are welded, or wiring is loose—the entire cooling cycle can be disrupted. This article explains how relays function in air conditioning, common failure modes, how to diagnose them, and when to call a professional.
How relays function in an air conditioning system
Most residential AC setups use a low-voltage control circuit to trigger higher-current devices. The thermostat sends a 24V signal, which energizes a relay or the indoor control board’s relay, closing a circuit that powers the outdoor contactor and/or the indoor blower. The outdoor contactor then feeds 240V to the compressor and condenser fan. If the relay coil is damaged or the contacts are pitted, the signal won’t reach the compressor or fan, and cooling won’t occur.
Common relay failure scenarios
Before listing common failure modes, it’s helpful to understand what can go wrong inside a relay or contactor. The following issues frequently lead to an AC that won’t start or run properly:
- Coil failure (open circuit or shorted coil) that prevents energizing the switch.
- Contacts welded shut or stuck closed, causing the compressor to run continuously or not at all.
- Contacts sticking open or pitting from arcing, leading to poor or no power transfer.
- Loose, corroded, or damaged wiring that disrupts the 24V control signal or the high-voltage feed.
- Overheating or age-related wear that degrades performance over time.
- Faulty thermostat wiring or a failing control board that never sends the proper signal to the relay.
Relays in the thermostat itself can also fail, leaving the outdoor unit without a proper call for cooling. In all cases, diagnosing the exact cause requires careful testing of both the control signal and the power path to the compressor and blower.
How to diagnose a relay problem
Diagnosing a relay issue involves both observation and measurement. This process can be done by a confident DIYer, but you should prioritize safety and consider hiring a professional for any work on high-voltage components.
- Turn off power to the system at the breaker to prevent shock or equipment damage.
- Set the thermostat to cool and listen for a click at the outdoor unit when cooling is requested—this indicates the relay/contactor is attempting to energize.
- Check for 24V at the outdoor contactor coil using a multimeter. If there is no 24V when cooling is requested, the issue may lie with the thermostat, control board, or wiring, not the coil itself.
- Inspect the outdoor contactor and its contacts for signs of wear, pitting, or welding. Burn marks or melted insulation indicate replacement is needed.
- Verify the outdoor unit has power (240V) across L1 and L2 when the coil is energized. If power is present but the unit doesn’t respond, the contactor or another component may be faulty.
- If safe, swap the suspect relay/contactor with a known-good unit to see if the system starts and runs normally. This can help confirm a relay issue.
- Check for ancillary causes, such as a blown fuse, a tripped breaker, a faulty transformer providing 24V, or a safety switch (like a high-pressure switch) that could prevent operation even if the relay is good.
Diagnosing relay problems requires careful measurement and an understanding of both low-voltage control circuits and high-voltage power circuits. If you’re unsure about any step, contact a licensed HVAC technician.
When to replace or call a professional
Replacing a relay or contactor is often a straightforward, cost-effective repair, but some situations require professional intervention. Consider professional help if you encounter any of the following:
- You're not comfortable working around high-voltage electrical components.
- The outdoor unit is not receiving 24V at the coil when cooling is commanded.
- You observe burning smells, melted insulation, or visible damage to wires or the relay/contactor.
- The refrigerant system may involve leaks, or the issue could be coupled with a failed compressor, which requires licensed handling.
- There is no consistent 24V signal after troubleshooting basic thermostat and wiring issues.
In many cases, replacing a faulty relay or contactor is inexpensive and quick, but persistent issues or refrigerant-related faults should be handled by a certified technician to ensure safety and code compliance.
Relays vs. contactors: what you need to know
What they control
Relays in HVACs typically switch low-voltage signals or small loads, while contactors handle high-current loads feeding the compressor and condenser fan. In residential systems, the thermostat often energizes a low-voltage relay inside the air handler, which then energizes a 24V coil on the outdoor contactor to power the compressor.
When to suspect each
If you hear a distinct click but the outdoor unit doesn’t start, the issue may be a failing contactor or the 24V control path. If you don’t hear a click or see 24V at the coil, the problem may lie with the thermostat, transformer, or control board rather than the outdoor contactor itself.
Summary
A relay can indeed cause an air conditioner not to work by failing to energize the compressor or blower, welding contacts, or sustaining loose wiring. Diagnosing the problem involves checking the control signal (24V), listening for system clicks, inspecting the contactor’s contacts, and verifying power to the outdoor unit. While some relay-related fixes are straightforward for experienced DIYers, many scenarios require a licensed HVAC technician to ensure safety and proper handling of electrical and refrigerant systems. Regular maintenance and timely replacements of worn relays or contactors can help prevent unexpected outages.
