Newb here looking for clarification. Have an old Lennox G12 furnace with an intermittent pilot and Johnson G60 controller. The igniter causes spark, but the flame does not always light. It only lights occasionally and then more often when warm (after first running in the morning). The furnace tech said he suspected the controller, but couldn't explain why it didn't light even when it was sparking.
The electrode gap has been checked, the flame shroud cleaned and the pilot orifice is clear. Anything else to check - or is it the ignition controller it?