Working on a york rtu m#d8cg060n09925a. The call came in that fan stays on. could not get unit to do it so the main control board was replased. week latter thay called again this time i went out and the fan was on. so i went to the unit and it was flashing 9 times limits all ok checked. turned unit on by jumping r w. heat came on. stopped the call for heat fan still stays on. switched out ignition control board from other unit on roof worked ok at that time. got call today that doing samething. there is 12 other units on roof all ok what we know of.
by the way the 9 flash is ignition contol lock out. butt the heat works fine.
has anyone have this and what did you do to fix it.
and if you turn power on and off the fan stops and the fan may work fine for sometime and it will stay on till thay turn power on and off.

If it has a flame sensor clean it or replace it and the cable.

Check all ground for tight connections. Bad grounds cause many problems with circuit boards.