Found problem - Coolant in transmission fluid

[GTOKID]

UPDATE - I flushed the trans and radiator.....speedo and 4th gear still not working. Bought a new output shaft speed sensor...did not fix problem. Connected up the old sensor and spun the sensor gear with a drill...the speedo moved. The gear inside the trans is not turning the speed sensor gear. Time to bring to Chevy.
They had to pull the trans and replace the differential assembly. Good news - no charge. Bought breakfast foe the whole shop.
MERRY CHRISTMAS.

[2010AveoLT]

UPDATE - I flushed the trans and radiator.....speedo and 4th gear still not working. Bought a new output shaft speed sensor...did not fix problem. Connected up the old sensor and spun the sensor gear with a drill...the speedo moved. The gear inside the trans is not turning the speed sensor gear. Time to bring to Chevy.
They had to pull the trans and replace the differential assembly. Good news - no charge. Bought breakfast for the whole shop.
MERRY CHRISTMAS.

I suspect what saved your Wallet was that your Powertrain Warranty was still good.