Q: Heater still blowing cold air?

When cooling systems are re-filled (after thermostat replacement for example) air can get trapped and that trapped air can prevent normal circulation of coolant. That, and low coolant level, would be the first things to check. If no problem is found there, the other common causes of no cabin heat include malfunctioning blend door(s) (part of the in-dash ventilation network), a faulty or stuck heater control valve, a plugged heater core, stuck open engine cooling thermostat (you already addressed this), and collapsed hoses. If you desire a diagnostic to determine exactly why there is no cabin heat, please request a non functioning cabin heater diagnostic and the responding mechanic will come right to your location and get this resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.