First off I am fresh out of school so pretty green but would like a few opinions on a furnace I'm trying to get going for a friend.
Lennox 90% Furnace starts up normally but flames get sloppy within a few seconds then go out and starts over. the code says flame sensor but the flame on burner the sensor is on gets sloppy very fast and not sure sensor has time to heat up. I have cleaned it with brillo, same outcome and sensor looks really good. combustion air switch checks out good.

orderd new flame sensor so figured I would run by y'all while I wait.

Thank you for your time, Scott