Performed heat check on two systems. I cleared the condensation drain lines and flow tested the drains with two gallons of water. I also treated the drain lines. The air filters were replaced with client supplied filters. On the system for the master bedroom, I found that the flame sensor was corroded. Options for repair were presented to the homeowner and he chose to have the flame sensor replaced. I replaced the flame sensor and function tested the system. No other major issues were found. The systems were running at the completion of service.