Furnace repair. No heat/furnace not keeping up. Cleaned and serviced furnace. Customer had one last visit on plan. Tested all mechanical parts. Found hsi and flame sensor very corroded. System was also dropping pressure after a while of running. Cleaned ptrap and all tubes and also inspected flue and also cleaned sand/mud from outside of exhaust pipe. System is heating and keeping pressure as it should now at this time. Customer replaced hsi and flame sensor. Giving customer estimates for new furnace due to being 21 years old, Inspected sump pump and recommend replacing battery back up systems for pumps.