Furnance will not ignite piot is on?

If the furnace won't ignite but the pilot light is on, the issue may lie with the thermocouple, which could be faulty and not allowing the gas valve to open fully. Additionally, there may be a blockage in the burner or a problem with the ignition system itself. Inspecting and cleaning the burner assembly or checking the electrical connections could resolve the issue. If these steps don't work, it's advisable to contact a professional technician for further diagnosis.