Most modern furnaces have a control board with diagnostic lights. This control is commonly located either in the furnace blower compartment or in the furnace combustion compartment. BEFORE you remove furnace doors check to see if there is a view port on the blower door. If so look through to view the furnace control.
Is there and LED illuminated or flashing a code?
If there is a LED light or a code flashing, record this information before opening the furnace doors. When the blower door is opened power to the furnace controls will be automatically shut off.
If the furnace does not attempt to operate:
Check thermostat for proper temperature. The thermostat must be set above existing room temperature to initiate furnace operation.
Check to ensure that furnace is in HEAT or AUTO mode.
If there is NOT and LED light illuminated, ensure that power is on to the furnace. Check the furnace emergency switch is on and check any external furnace fuses.
If the furnace attempts to operate but fails to start properly:
This means that the furnace has power, and is getting a heat signal from the thermostat, but is not lighting properly. Most commonly the furnace will attempt ignition several times and then lock-out.
Check gas supply. Check that the manual gas valve shut-off in the gas pipe leading to the furnace is OPEN.
To proceed further TURN OFF ELECTRICAL SUPPLY TO THE FURNACE!
Check the control switch on the gas valve. Is it in the ON position?
Check for manual reset switches in the burner compartment. Attempt to reset manual roll-out or limit switches and attempt to re-start.
Check the condition of the vent. If the furnace has a traditional chimney vent check to ensure the condition of the vent. If the furnace vent is plastic and terminates horizontally, check that the vent pipe is properly supported, and that there are no obstructions visible at the exterior vent termination like snow or ice, debris, or insect nests.
If the furnaces operates but it cycles on and off or heats the space more poorly than when it was last serviced or installed:
Check the air filter, and replace or clean if dirty.
Check to ensure there are no obstructions to furnace airflow. Make sure that return registers are not obstructed, and that most supply registers are open.
Start at the thermostat. Make sure that it is set to cooling and that the thermostat setting is lower than the current room temperature.
If the thermostat is set properly and calling for air conditioning:
Go to the server of your floor, wall or ceiling supply registers. Is there air coming out of the register, and is it cold?
If there is no air coming out of the register:
Go outside to see if the fan on the outdoor unit is running. If the outdoor fan is not running, check the sure circuit breakers to ensure that they are not tripped.
If the breaker is tripped:
You can reset it by turning it off and then back on. If it trips again a service call is warranted.
If your unit has a condensate pump:
Check to make sure the reservoir is not full. If the reservoir is full, a limit switch will normally stop the air conditioner from running. You will need to repair or replace the pump.
If at this point the breaker and condensate pump are ok and nothing is operating:
Then the problem is probably the thermostat, the system control board (in the furnace or air handler, or the wiring between the indoor unit, the outdoor unit, and the thermostat.
If the indoor blower is operating but the outdoor fan is not:
Check to see if there is an outdoor disconnect, and that it is set properly.
If the indoor and outdoor fans are on:
Check the refrigerant lines at the indoor coil. Do you see heavy frost or ice on them? If there is frost, check the for proper air-flow. Is the system filter clean? Replace or clean it if necessary. Also, check that the return air grills are not obstructed and that the majority of supply registers are open.
If the refrigerant lines are not iced:
Check the outdoor unit coil. Is it relatively clean? Ensure that if is not obstructed by foreign material like leaves or pollen.
If the system seems to be operating, but outdoor conditions are extreme:
The unit may not be large enough to maintain a desired indoor conditions. During extreme conditions try to avoid activities that add to the thermal demands of your system including: clothes drying, longer showers, oven use, or frequent opening of exterior doors.
If the unit operates and the airflow has been checked, but the cooling is inadequate or the refrigerant lines are iced a service call is warranted.