No heat coming out , I was told I needed a servo sensor on 1999 Lexus RX300

I was driving the car last night and the check engine light came on... I don't know whether I should just take it to Advance first to see what the codes are or go to the dealer directly..

Asked by for the 1999 Lexus RX300
Does the engine reach operating temperature? What's the temp gauge showing after you drove it for a while? It may be just a thermostat and the CEL relates to the long open loop mode.
The engine warms up and everything but when you turn the heat on, turned it all the way to HOT, there was no hot air coming out of the vents..I took it a shop to see if they could put in a new thermostat and that was the answer I got.. that I needed a servo sensor ... Is that an accurate answer? Should I take it to the Lexus dealer to be fixed? Plus now, the check engine light is on.. :( Thank you for your reply...