When your HVAC fan won't stop running, the issue typically stems from thermostat settings, electrical malfunctions, or component failures within the system. This persistent operation not only increases energy consumption but may also signal underlying problems that could worsen without intervention. Most cases begin with simple checks like verifying thermostat settings or resetting the circuit breaker, but some scenarios require professional diagnosis to address faulty wiring, malfunctioning relays, or damaged control boards.

Key immediate actions to consider:

• Check thermostat settings – Ensure the fan is set to "auto" rather than "on" ^[2][8]
• Reset the system – Turn off the breaker for 30+ minutes or unplug the indoor unit to reboot the system ^[4][6]
• Inspect for electrical issues – Look for tripped breakers or damaged thermostat wiring ^[3][5]
• Evaluate component failures – Faulty relays, sequencers, or ECM motors often require professional repair ^[3][10]

For persistent issues, consulting an HVAC technician is critical, as electrical problems or internal malfunctions (e.g., bad circuit boards or shorted wires) pose safety risks and can damage the system further ^[1][7].

Step-by-Step Troubleshooting for an HVAC Fan That Won’t Stop

1. Thermostat and Basic Settings Checks

The thermostat is the most common culprit when an HVAC fan runs continuously. Misconfigured settings, faulty wiring, or a malfunctioning unit can all prevent the fan from cycling off as intended. Start with these verifications before exploring mechanical or electrical issues.

Begin by confirming the fan setting on your thermostat. If it’s set to "on" instead of "auto", the fan will run indefinitely, regardless of whether the heating or cooling cycle is active ^[2][8]. Switching to "auto" often resolves the issue immediately. If the setting is correct but the fan still won’t stop, proceed to deeper diagnostics:

• Battery and power issues:
• Replace thermostat batteries if the display is dim or unresponsive. Low power can cause erratic signaling ^[8].
• For hardwired thermostats, check if the system has power by verifying the circuit breaker hasn’t tripped ^[5].
• Wiring problems:
• Inspect thermostat wiring for loose connections or visible damage. Shorted or corroded wires can send continuous signals to the fan relay ^[3].
• If comfortable, turn off the HVAC breaker and remove the thermostat cover to visually check wires. Do not attempt repairs without HVAC experience ^[1].
• Thermostat malfunction:
• Test the thermostat by setting it to a temperature significantly higher (for cooling) or lower (for heating) than the current room temperature. If the fan ignores these changes, the thermostat may need recalibration or replacement ^[10].
• Try resetting the thermostat to factory defaults (consult the manual for steps). Some modern thermostats require a full system reboot via the breaker ^[6].

If these steps fail, the issue may lie beyond the thermostat. Proceed to electrical and component inspections, but avoid disassembling the HVAC unit without professional guidance.

2. Electrical and Component-Level Failures

When basic checks don’t resolve the issue, the problem often stems from electrical failures or damaged internal components. These scenarios typically require technical expertise, but you can perform preliminary troubleshooting to identify potential causes.

Common electrical and mechanical culprits:

• Faulty relay or sequencer:
• The relay controls power to the fan motor. If it’s stuck in the "closed" position, the fan will run continuously. Listen for a clicking sound when the system cycles—absence of this noise may indicate a bad relay ^[3].
• Sequencers (in older systems) can develop burnt contacts that remain closed. This requires replacement by a technician ^[3].
• ECM motor or control board issues:
• ECM (Electronically Commutated Motor) blower motors have integrated control boards. If the board fails, the motor may default to continuous operation. This often necessitates professional diagnostic tools ^[3][10].
• Symptoms include the fan running at a single speed (even when variable speeds are expected) or ignoring thermostat commands.
• Fan limit switch problems:
• Located on the furnace, this switch regulates fan operation based on temperature. If set to manual override, the fan will run nonstop. Reset it to "auto" if accessible ^[8].
• A faulty switch may also fail to shut off the fan after heating cycles. Testing requires a multimeter and HVAC knowledge.
• Short circuits or damaged wiring:
• Shorted wires in the thermostat cable or control board can bypass normal operation. Inspect for burnt smells or scorch marks near the air handler or furnace ^[1].
• If you suspect a short, turn off the HVAC breaker immediately and contact a technician. Electrical fires are a serious risk ^[5].

Immediate actions for suspected electrical issues:

• Turn off the HVAC system at the breaker before inspecting any components ^[5].
• If the fan stops after power cycling (turning the breaker off/on), the issue may be intermittent. Monitor for recurrence and schedule a professional inspection ^[4][6].
• Avoid attempting repairs on high-voltage components (e.g., capacitors, transformers). These pose shock hazards and often require specialized tools ^[1][3].

For problems involving relays, control boards, or motors, professional repair is strongly advised. These components are sensitive, and incorrect handling can void warranties or cause further damage ^[7][10].