I recently had this problem and it ended up being the "Map Sensor". An $80 part. Its odd that the Map sensor would have anything to do with the transmission not wanting to shift but its actually a safety feature. If you go to the website Answers, you will find a great explanation about the flashing HOLD light. Its under "What does it mean when Flash hold light in chevy aveo" Its on page 2. Its a good read.
