Hi experts,

I have a 14 years old York Diamond 80 furnace. Starting about 2 or 3 years ago, it blows heat for about 20 minutes and continue blowing room temperature air. Shut off the thermostat or any thing in the thermostat will not turn off furnace blower. When the furnace starts blowing cold air, it flash 8 in led. The only way to shut off the furnace is turned off power. Once the power is turned back on it will works fine if the set temparature is reached within 20 minutes. Otherwise, the same problem will appears again. I have clean flame sensor, changed flame sensor and roll-out switch. But the problem is still there. Filter is changed regularly.
I am in California and the temperature is not that cold. So I can turn off the furnace in thermostat to make sure the furnace does not blow more than 20 minutes a time as a work around. The only time that runs more than 20 minutes is in a cold morning and I do not really want to go into cold garage in the morning to reset power of furnace every morning. Would you please help me to diagnose and fix this problem?
Thanks in advance.