Performed AC cool check. Checked electrical, mechanical, safety, and performance of the system. I cleared the condensation drain line and flow tested the drain with two gallons of water. Clear water was observed flowing out of the drain line. I also treated the drain line. The air filter was replaced with a customer supplied filter. The drain line is piped without a vent to help with water flow and has a trap made from 90-degree PVC connections. The drain is draining now but has had issues in the past. Some mold/mildew was found growing inside the air handler. No other major issues found.