Arrived on site to diagnose furnace not working. Found that the furnace would stay lit for a few seconds and turn off. Inspect igniter and flame rod. Both looked good but the burners were very dirty. We removed the burner to inspect the heat exchanger and found evidence of mice in all three tubes. We believe there could be more deeper inside and that is causing the pressure switch to trip and turning the furnace off. Due to not being able to remove the mice from the heat exchanger we recommend either replacing the heat exchanger or the furnace altogether. There was also mice carcasses in the blower compartment. We turned the gas and power off to the unit and will be providing quotes for furnace and water heater replacement. Thank you for your business!