| Cool air in heat mode | Thermostat on Cool, Auto or Off, dead thermostat batteries, a stuck reversing valve, a control fault, or low refrigerant. | Set the thermostat to Heat (not Auto) and above room temperature, and replace its batteries if the display is blank. | Air is still cool several minutes after the thermostat calls for heat. |
| Weak heat or very long run times | A dirty filter, closed vents or blocked returns, a dirty coil, low refrigerant, or backup heat strips that are not working. | Clean or replace the filter, open supply vents, and clear furniture away from return grilles. | Heat stays weak with a clean filter and open vents. |
| Outdoor unit covered in ice | Light frost is normal and the defrost cycle clears it; heavy ice points to a defrost control or sensor fault, poor airflow, or low refrigerant. | With the system off at the thermostat, clear leaves and debris from around the outdoor unit. Never chip at the ice. | Ice keeps building up or the coil stays encased after a defrost cycle. |
| Stuck in cooling mode | Thermostat mode setting, the reversing valve or its solenoid, or the control board. | Confirm the thermostat is set to Heat, not Cool or Auto. | The mode is set to Heat and the system still cools. |
| Short cycling | A dirty filter, a thermostat near a supply vent or in direct sun, an oversized system, or a control fault. | Replace a dirty filter and check where the thermostat sits. | It keeps turning on and off every few minutes after those checks. |
| Burning smell or a tripped breaker | Backup heat strips, a failing capacitor or motor, or a wiring fault. | Turn the system off at the thermostat. Reset a tripped breaker once, only if there is no burning smell. | Any burning or electrical smell, or a breaker that trips again: leave the system off. |