Universal diagnostic library
HVAC troubleshooting by system and observed state
HVAC troubleshooting by system and observed state: resolve equipment, model, thermostat demand and indicator before treating a safety control as failed.
Heating and cooling faults are best sorted by what the equipment does. A thermostat screen that is blank is usually dead batteries or a tripped breaker or float switch; a furnace that starts and stops every few minutes may be overheating from a dirty filter, or shutting down because the flame or pressure switch is not satisfied; an air conditioner that runs but does not cool needs its filter, indoor coil and outdoor unit checked for dirt and ice. A furnace in lockout has shut itself down for safety, and a heat pump on auxiliary or emergency heat is using electric backup that costs more to run.
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7 complete records in this directoryAir conditioner runs but does not cool the house
Prove thermostat demand, indoor airflow and outdoor-unit operation before considering refrigerant or electrical components.
Open record Furnaces · lockoutFurnace lockout: hard lockout, soft lockout and what resets it
What a furnace lockout is, the difference between a soft and hard lockout per Trane, how long Carrier and Lennox lockouts last before the control retries, and the only fix Trane says to try yourself.
Open record Furnaces · no heatFurnace runs or calls for heat but the house stays cold
Work a gas furnace that will not heat in the order Trane and Carrier use: thermostat, power, filter and airflow, gas supply, then the status light. Includes where the Carrier and Lennox code lists are and when to stop.
Open record Furnaces · cyclingFurnace starts and stops every few minutes
Trane says a normal furnace cycle lasts at least 7 minutes. Find out whether yours is short cycling, the most common causes in Trane's order, and which ones are safe to check yourself.
Open record Heat pumps · auxiliary heatHeat pump auxiliary or emergency heat stays on
Auxiliary heat may be normal in cold weather, defrost or large temperature recovery. It is suspicious when it remains active in mild weather after setpoint is reached.
Open record Thermostats · powerThermostat screen is blank
A blank thermostat can result from batteries, HVAC service power, a condensate float, loose seating or loss of low voltage. Do not jumper wires.
Open record HVAC · condensateWater is collecting around the furnace or air handler
Shut cooling or condensing heat off when water approaches wiring. The source may be a blocked drain, pump, frozen coil, humidifier or plumbing.
Open recordHeating and cooling problems before you call a technician
Safe checks include replacing the air filter, making sure vents and returns are open, checking the thermostat mode and set point and that the breaker and furnace switch are on. Water around the furnace or air handler is often a clogged condensate drain. Gas smells, burning smells, repeated lockouts and anything inside the equipment are for a licensed technician. The pages on furnaces, heat pumps and air conditioners go through each system in detail.
Why does my furnace start and stop every few minutes?
A dirty filter that overheats the unit and a safety switch that cannot prove airflow or flame are the common causes. Replace the filter and check the vents first.
What does furnace lockout mean?
The furnace has shut itself down after repeated failed starts for safety. It can be reset once at the thermostat or power switch; if it locks out again, call a technician.