Trans slips only on left turns. Will a new trans

change filter ,fluid and gasket --fresh filter and fluid may save ypu if the transmission is not totally burnt up also a additive like lukas oil helps the following link shows how to change filter in an impala it is easier to drain the trans by putting a pan under and loosen the bolts and leave 2 in and pry opposite side and slowly tilt pan down to drain than remove all bolts and clean out pan
04 Impala Thranmission Filter Yahoo Video Search Results

Hello tat... begin by checking the tranny fluid level. Add fluid to fill it to just over the max full level.

Drive it for ten miles and see if that alone solved the problem. IF not, have it serviced and have the belts tightened. Joe

The clutch apply seals may have hardened and the additive is just a patch for now.

if never done do do it.
the trans will slip guaranteed

no but a low fluid level could cause a slip telling the ecm there is a slip top it off then clear the code and drive a few miles if this code comes back i would say there is internal damage to the tranny

Could be low on compression, or torque converter may be slipping

Did you check the trans oil level. When you changed
the RADIATOR you will loose some trans oil.
If the transmission oil was low when you changed
the radiator it's lower now. Check it now.
Now, if thats not the problem. With that many miles
have you changed the trans oil lately theirs a filter
above the trans oil pan that may be pluged up change
it and the oil. Also, buy a can of trans X it's the only fix
in a can stuff I"LL use.

theres not much you can do about a slipping trans. its probly more noticable when you turn left because your left rear wheel is your primary drive wheel, it takes more force to move your car from the inside corner than the outside. be gentle on it and it may last you a while

Have you checked the fluid? It sounds like its low. If its fine, change the filter.

there are several issues that need to be addressed. I imagine that the trans oil and filter have not been changed in a while so that is a first step in fixing the problem. trans oil and filter should be changed every 50,000 miles for atf oil and 75,000 for synthetic oil. old trans oil and clogged filters are #1 reason for trans slippage. another reason is the trans controle cable is out of adjustment and there fore the trans doesn't shift like it should this is something that needs to be looked at by a mechanic. third possibilty is either the check valves inside the trans valve body are gummed up or you have a failing torque converter. try and get that oil and filter changed and see if this helps solve it if not than you need to takeit to a shop and have the trans rebuilt.