Completed maintnenace on 3 systems. Upstairs unit has mold in the supply plenum, igniter has very high resistance and could break any time, transformer is over heating and can cause issue in near future, blower wheel is dirty and duct work is dirty. Main floor has very rusty evaporator coil. Basement system transformer is over heating can cause issue in near future. Replaced transformer on upstairs and basement system. Units are all heating normal at this time.