If you're mechanically inclined, you can pull out a service manual ($10 download @ http://techinfo.toyota.com) and buy a scan tool ($200 Auterra plus a PDA) if you want to fix this yourself. Pull the Diagnostic Trouble Codes with the scan tool and find which O2 sensor is bad, and after testing it and finding it to be at fault, you could buy an O2 sensor from the stealership for $180 retail and change it out youself. Then you'd have a scan tool, service manual, and $200 in your pocket for more tools to work on your car if it has problems ever again.
I won't let a dealership touch my cars. They reduce the life of a car by 50,000 mi every time they service a vehicle.