Lennox Furnace - clicking and won't light

[gstaff]

I just replaced the thermostat for my system- replaced old Honeywell with a newer Honeywell - both have identical lead screws on the back, and I have checked dip switches, etc.

The A/C works, as does the manual fan control, but the Lennox heater (m/n 8OUHG475A2) will not start. I hear clicking, but no luck with the restart. I closed the unit's gas valve for 5 minutes and then tried again - no luck.

Could the thermostat be bad, or do I have a pilot problem?

Thanks.