A few months back I had the duo valves replaced because I wasn't getting cooling and the middle front vents were not thermally "equalized," and there was that hissing also. Fast forward today: Ambient now is in low 50s (F). No air coming from middle front vents in normal temperature, say 72F. Duo valves "thump" in no consistent fashion that I can speak of. If I set to LO, the vents open and I get quasi-warm air, duo valves stop thumping. I tried to run the diagnostics - can't get past the portion where the red flashing light comes on, so I can't get the codes. Help!