2009 impala ss need help

Could be a shift solenoid but usually winds up being a broken reverse band.

If your transmission works properly in all forward gears but not in reverse, the problem could be that the transmission fluid is too low for it to function correctly. It's also possible that the fluid is dirty and has lost its ability to do its job. In that case, you likely have other transmission issues caused by this and need to see a transmission specialist..... Change your fluid and if it's dark in color, your issue might be overheating, which can cause other defects, as well. When you shift an automatic transmission into reverse using the shift lever, the transmission position sensor tells the PCM to shift the transmission into reverse. If it doesn't, the sensor might be faulty. Also, a worn valve body might leak and create opposing pressures that inhibit shifting into reverse.