Came out to call for a system not heating properly. Customer stated that they had issues with the heater and had a family friend come out this weekend that has experience in HVAC. He let them know what was possibly the issue, but the system was operating, up until today. Customer had system turned off at the thermostat, so I had her turn it on. I checked system operation and found it working properly, but in need of some maintenance. Flame sensor was dirty, and so were the burners. Pulled blower and inspected the heat exchanger and evaporator coil. No failures in the heat exchanger of what I could see and the evaporator coil was clear of many major build up. Told customer I would clean necessary components and test system to verify it’s operating properly. Customer also had a thermostat that I helped hook up. I cleaned the flame sensor and dust off the burners. Checked amps, temperatures and gas pressure of system. Systems temperature rise was within range and no immediate mechanical f