Kia Troubleshooting Questions? Ask a Mechanic
First, the mileage on the sticker is an estimate only based upon carefully controlled testing on a dynometer or closed track. The highway mileage is based upon steady speed of 55mph with no braking for traffic and no acceleration to pass other vehicles. These are the standards the NHTSA uses for all vehicles when getting mileage ratings.
As to using a chip, it will indeed void your warranty as it does tamper with engine management and emissions systems.
You can try to get Kia to replace the sensor, but unless it tests bad, they will not replace it under warranty. They have to send every part replaced under warranty back to Kia and it is tested there. If it tests good, the dealer is charged back for it.