Blows warm air only at low speeds Is it the thermostat?

Yes, the thermostat could be stuck open.

. Ensure the cooling system is completely full of coolant and that there's no entrapped air.

. Ensure heater hoses are not kinked or collapsed.

. Check if the inlet and outlet lines to the heater core are hot, when the heater is turned to high. This tells you that coolant is circulating properly. If both lines are not hot, the heater core may be blocked, due to corrosion or phosphate scale, or the heater control valve may not be working properly. In this case, reverse flush the heater core. Afterwards, refill with a 50/50 mixture of manufacturer recommended antifreeze and distilled water. If the heater core is plugged, and the flush and fill does not solve the problem, then it needs to be replaced.