Arrived on site to pet tune up 2 5 year old lennox units. Client stated he was as having issues upstairs and wanted to diagnose upstairs as well. Cycled both heaters on and both turned on. Inspected upstairs unit after a couple minutes unit shut off due to no flame sensed. Unit tripped safety switch. Pulled and found flame sensor really oxidized and corroded from burning up in the past years. Cleaned sensor and unit was operational and. Did trip at this time. advised client flame sensor is running at this time but recommend replacing. Performed tune up. Filter are dirty and client has a purification kit on both units. He has yet to replace maintenance kit on both and has just been doing filter. Recommend replacing maintenance as they are not operating as they should. U.V light is very dim. Both blower assembly and wheels are good. Inducer motor are both good and in range. Both igniters are in range. Both flame sensors are oxidized and not good. Recommend replacing. Drain lines good at