[Qinathan]

York furnace, Model GY9S060B12UP11G, is not working with four rapid amber flash code. Reset the door switch and the fault code comes back immediately after the green flash without any action of the control board and the motors. Found the air filter was very dirty and was changed. The problem stays on. Please tell. Thanks.

[drtom4444]

How clean are the coils and the blower wheel? More than likely that is your problem: low air flow. I would call a good HVAC mechanic and have it cleaned, which they hate to do, but it needs cleaning every year. Cleaning means removing coils and blower and pressure washing them after spraying on coil cleaner. Sometimes you can clean coils without removing them in package units and large commercial units, but usually removing them is the best way to do it. DrTom4444

[Qinathan]

Dr. Tom444, Thanks. Finally, I got the answer by resetting the pressure valve. I figured out the reason that the pressure built up inside the valve due to the clogged filter and sent warning to the control even after started it.

[drtom4444]

That would be the low pressure switch. You usually don't see those except on commercial units. Still, you need to check the coils, and go by what they may look like on the outside. Look on both sides and shine a light through it and look in drain pan. If it is dirty clean the coils. You will be surprised at the dirt that will foam out of them. DrTom4444

[drtom4444]

I meant to say do not go by what they look like on the outside. DrTom4444