Performed preventative maintenance on 3 furnaces. Inspected blower assembly, blower motor, electrical components, heat exchangers, burners, flame sensor, ignitors, gas valves, inducer motors and overall condition of equipment. The two Amana units have some wear on the heat exchangers, but no cracks found at this time. The u it for the downstairs part of the house was making some vibration noise on start up. It was the inducer motor making the noise. The amp readings were within range, but made homeowner aware of the potential bearings wearing out in that part. Customer will call if they want to replace it. I cleaned the flame sensors and brushed dust off the components. Checked thermostat operation, wiring, capacitors, amps, temperatures and gas pressures. All 3 systems are still heating properly within manufactured specifications at this time.