Heater blows cold air

Assuming you mean that the ventilation system blows cold air even when set to 90 degrees, as in your subject line. If there is cold air when set to a hot temperature, the most common cause is the temperature blend door operation in the dash not turning properly to meter the air to the hot position. This can be due to a control issue, a motor issue, or a fractured heater case or blend door. If the engine is not getting hot enough, you won't get heat either -- so the first thing to do would be to check for proper engine operating temperature before moving onto climate control system function.