Upon arrival, inspected system and found system operational at this time. Open unit up and unit was excessively dirty with dust and debris. Removed spark igniter and flame sensor, wire brush, each component and reassemble after vacuuming whole burner and blower compartment. Start furnace back up and test run system. System is operational at 48° delta T. Homeowner was asking about thermostat going into temporary. Advised her to use the hold function on the thermostat in order to override scheduling. Unit is heating at this time.