Are the limits and roll out switches closed
furnace sets code for roll our switch or limit open. Furnace locks out until switch closes or if furnace locks out on set amount of tries during 1 heat cycle
Furnace control board starts inducer
are the pressure switches closed
after a set amount of time the furnace locks out on pressure switch fault. usually the inducer will continue to run for either set amount of time or will continue to run until switch closes.
ignition board closes relay in board to energize the ignitor.
after a set amount of time the ignition board closes a relay in the board and sends 24v to the gas valve.
the ignition board send usually between 90v -110v through the flame sensor and looks for a set amount of micro amps through the flame sensor. usually 3-5 U.A.
did the board sense the required micro amps?
after a set amount of time the ignition control board locks out on flame failure
furnace continues to run until thermostat is satisfied
Sequence of operation and likely causes of faults.
W call from thermostat
is the pressure switch closed before inducer is on. (Some furnaces check t see if the pressure switch is closed and locks out on pressure switch closed before inducer call but some do not monitor this.)
no
yes
if pressure switch is closed need to replace pressure switch if inducer is not not running. If inducer is running need to check for reason inducer running before heat call.
