Yellow flame all the way down the heat exchanger. Checked gas pressure set right at 3.5” wc, temp rise is a good 50 degrees, ran analyzer to find ppm spike at start up to only 250-300ppm and then leveled out around 30ppm. Cleaned flame sensor, vacuumed blower and inducer cabinets, both motors amp draws are in spec, spark ignition had a clean fire off, order of operations happened in proper sequence, and system is now operating optimally with a new uv bulb in. Customer reported they changed their own humidifier pad.