I've had this problem with this car for along time. I'm a mechanic and have tried a new speed sensor in trans, checked grounds, and replaced the ecm. The car will run for weeks without and problems and then will set codes and the trans will default to second gear and the car clunks. I've talked to others and nobody really knows. I was thinking of trying a TCM and been told it might fix it but, don't want to spend 300 and not fix problem. I looked through this forum and did not see any other aveos with same issues. Any help would be great! Thanks