Lennox Pulse 21 tries to start but kicks back off??

Yes it has 3 AAA batteries.
Pull it straight off the wall. Look at the back of the thermostat and replace the three batteries.

Link to manual..
http://www.lennox.com/pdfs/manuals/Lennox_Elite_Touchscreen_Manual.pdf

You are using too restrictive of an air filter. If the codes you are getting off the lights says anything about a high limit this is the cause of your blower running all the time. Use a less restrictive filter or have your contractor install a media filter in the system. if you reset the furnace and it does not stop running the blower you have a stuck limit switch and may have to get another or call a service tech to reset it or replace.

Hello, seems like there is no way to turn on the humidifier. There would either need to be a new transformer installed, a current sensing relay, or a sail switch to activate the humidifier when the blower motor kicks on.

Hi it could be a faulty fan control it cuts in on temp rise bring the fan on / and shuts off if the temp gets too high if fan dose not cut in .

you can hook it to R and W on the board but you should really use a current sensing realy on the blower so it wont come on until the fan is running and you should also have a humidistat

It is possible that you did not set the new blower on the proper speed. Black wire is high. Blue is medium. Red is low. If the fan is operating to slow it can run to hot and trip the high limit control. Make sure the filters are clean. When the old motor went bad, the problem might not have been discovered rite away your high limit control might need to be changed.
Also anytime you work on a furnace you can kick up some dust. Always clean the flame sensor.
Good Luck
HVAC Teacher

The humidifier should not turn on untill the furnace blower starts. Since the fan stays on. does the solenoid valve also stay on? If they both stay on you probably have a bad humidistat. If only the blower stays on then the circuit bourd in the humidifier is defective.
Good luck
HVAC Teacher

ther might be dip swiches on the circuit board you might have to change

It doesn't sound like it has failed.

The whole point of the % setting is so that the room holds at that %. When the humidity drops lower than, it will kick in. When it raises to that number it will turn off.

If the on and off is bugging you try raising the percentage. Or lower the temperature of your house. Heating will dry out the air and cause your machine to kick in.

Good Luck.

I WOULD CHECK TO SEE IF YOU HAVE ENOUGH V/A OUT-PUT ON TRANSFORMER. I DONT KNOW IF YOURS CAME WITH ITS OWN TRANSFORMER. IF IT DIDNT YOU SHOULD HAVE A MINIMUM OF A 40 V/A RATING STAMPED OR PRINTED ON THE EXISTING TRANSFORMER. IF THE TRANSFORMER IS EXTREAMLY HOT ... YOU MAY NEED A 75 V/A TRANSFORMER WITH THE SAME VOLTAGES. INCREASING THE V/A CANNOT DAMMAGE ANYTHING IT JUST GIVES YOU MORE POWER WHEN EXCESIVE CONTROLS AND WIRING ARE ADDED TO A HVAC LOW-VOLAGE (24v) CIRCUIT. IF YOU LIKE YOU CAN CALL ME AT 727-363-7071. I AM A RETIRED HVAC&R CONTRACTOR WITH 45 YEARS EXPERIENCE. I HATE THE NEW SENSORS THEY HAVE TO ACTIVATE THESE NEW HUMIDIFIERS. I USE TO WIRE THE HUMIDIFIER TRANSFORMER (SUPPLIED WITH HUMIDIFIER) TO THE LINE (110V) SIDE TO THE LOW SPEED ON YOUR BLOWER MOTOR. THAT WAY THE ONLY THING THAT INTERUPTS THE CICUIT IS THE HUMIDISTAT. YOU NEVER WANT THE HUMIDIFIER TO RUN IF YOUR SYSTEM IS NOT "CALLING FOR HEAT". NEVER TRY TO ENERGIZE THE WATER SOLENOID THROUGH YOUR HEAT WIRE(USUALLY SMALL THE WHITE WIRE) FROM THE THERMOSTAT. THIS WILL RESULT IN BURNING YOUR HEAT ANTICIPATOR OUT ON THE T-STAT.