I have disconnected the unit at the power supply waited a few minutes and as soon as I power on inducer fan starts immediately no matter if its in heat or ac mode.

Try disonnecting all thermostat wires from the furnace control. If it still runs, check for any open high limits, or flame rollout switches. If it still runs, you have a stuck inducer fan relay, or a bad control board, all depening on where the power to the fan comes from . Trace the wires back from the inducer motor and see where they go. If to an individual relay, give it a good wack with a screwdriver and see if it will stop running.