2001 chevy impala transmission will not stay in gear

The neutral safety switch is a safety device that only allows you to start your engine when the automatic transmission is in either Park or Neutral. The purpose of the neutral safety switch is to prevent the car from starting while in gear, which would cause it to launch forward unexpectedly.
https://oards.com/neutral-safety-switch-symptoms-and-cost/

Use you hand lol. lefty loosey theory all the way one way or the other will be either 1 gear all the way opposite will be park

about replacing a transmission with one from a Junkyard or cross referencing them. and to show you that not all 4T65E transmissions are interchangeable, although they look alike. Guys with the comp g package or the Impala police option often find this out the hard way.
Your car and the potential donor car will have a tag in the trunk (usually under the spare tire cover) that lists the RPO codes for your car, along with your VIN. RPO=Regular Production Order. Several of these codes relate to the transmission, and you will need them to begin to find a proper match. If you can't find the RPO tags, you can look up the RPOs for a nominal fee ($5) according to the VIN number at the following website: www . compnine . com .
I know these transmissions are different in at least 5 areas, and you have to match each area for a proper swap.
FIRST is the transmission type, which relates to torque converter size, the strength of the internal gears, and the strength of the differential. Here are the basic types along with their RPO codes:
M15 - 245 mm (9.6 in) torque converter
MN3 - 258 mm (10.2 in) torque converter
MN7 - 258 mm (10.2 in) torque converter and heavy-duty gearbox and differential (see 4T65E-HD)
MD7 - 245 mm (9.6 in) torque converter (Chinese version)
M76 - 245 mm (9.6 in) torque converter (with all-wheel drive capability)
You will want a match these RPO codes.
SECOND is the final drive ratio. The PCM can detect the difference, and in some cars may not engage if the correct final drive is not present. Here are the RPO codes for the four different final drives used in the transmission.
FQ3 - RATIO TRANSAXLE FINAL DRIVE 2.86;
FR2 - RATIO TRANSAXLE FINAL DRIVE 2.93;
F83 - RATIO TRANSAXLE FINAL DRIVE 3.05;
FR9 - RATIO TRANSAXLE FINAL DRIVE 3.29;
To narrow year search for potential cars, the following site have pretty good info on the final drives used on various cars. Click on the years of the cars in question.http://www.gmtuners.com/transmission/index.htm
THIRD is the converter stall speed. There will be small differences for most cars as it is based on car weight as well as final drive value and engine ratings. Reusing the cars original torque converter will address that area. Some indication of stall speed value is given in the K-factor information in the above gmtuners page. This bottom of this page from the gmtuners website will show you how to decipher the four digit code on the torque converter. http://www.gmtuners.com/gmtransinfo.htm
FOURTH, the torque converter needs to have the correct lockup clutch friction material for the PCM that controls it (regardless of HD nor not). This changed over the years. The newer ones can be used on older transmissions with calibration changes, but older type in newer transmissions will fail. Reusing the cars original torque converter will address that area.
FIFTH is area is electrical compatibility, and this is what trips up a lot of guys. 3 different electrical interfaces to the transmissions were used from 97-08. Some of these differences had to do with how the PCM determined what gear the transmission was in, or was commanded to be in. If you look on a GM parts web site, you will see that three different internal harnesses are available for the 4T65E. Each of the three harnesses is listed below along with the cars that use the harness. Your car and the donor car will have to match. If you have any additional insight, please leave me a comment and I can correct this post.
CARS USING HARNESS 24241204
Buick Century 4DR Custom2000, 2001, 2002
Buick Century 4DR LIMITED2000, 2001, 2002
Buick Lesabre 4DR CUSTOM1998, 1999
Buick Lesabre 4DR LIMITED1998, 1999
Buick Park Avenue 4DR Base1997, 1998
Buick Park Avenue 4DR ULTRA1997, 1998
Buick Regal 4DR GS1998, 1999, 2000, 2001, 2002
Buick Regal 4DR LS1998, 1999, 2000, 2001, 2002
Buick RENDEVOUS 4DR2002
Buick Riviera 2DR Base1997, 1998, 1999
Chevrolet Lumina 4DR Base w/ 3.1L (LG8) eng2000, 2001
Chevrolet Lumina 4DR LTZ1997, 1998, 1999
Chevrolet Monte Carlo Z341997, 1998, 1999
Chevrolet Venture2001, 2002
Oldsmobile 88 4DR Base1997, 1998, 1999
Oldsmobile 88 4DR Regency1997, 1998
Oldsmobile 88 4DR Royale, Base and LS1997, 1998, 1999
Oldsmobile 88 4DR ROYALE LSS1997, 1998, 1999
Oldsmobile Intrigue 4DR Base1998
Oldsmobile Intrigue 4DR GL1998, 1999, 2000, 2001, 2002
Oldsmobile Intrigue 4DR GLS1998, 1999, 2000, 2001, 2002
Oldsmobile Intrigue 4DR GX1999, 2000, 2001, 2002
Oldsmobile Silhouette1999, 2000, 2001, 2002
Pontiac Aztek Hatchback Base and GT2001, 2002
Pontiac Bonneville 4DR SE1998, 1999
Pontiac Bonneville 4DR SSE1997, 1998, 1999
Pontiac Grand Prix 2DR GT1998, 1999, 2000, 2001, 2002
Pontiac Grand Prix 2DR GTP1999, 2000, 2001, 2002
Pontiac Grand Prix 4DR GT1998, 1999, 2000, 2001, 2002
Pontiac Grand Prix 4DR GTP1999, 2000, 2001, 2002
Pontiac Grand Prix 4DR SE2000, 2001, 2002
Pontiac MONTANA Base1999, 2000, 2001
Pontiac MONTANA EXT MOBILITY SPECIAL2002, 2003
Pontiac MONTANA MPV1999, 2000, 2001
Pontiac MONTANA MPV BASE2002, 2003
Pontiac MONTANA MPV EXT1999, 2000, 2001, 2002
Pontiac MONTANA MPV EXT BASE2002, 2003
Pontiac MONTANA MPV EXT UPLEVEL2002, 2003
Pontiac MONTANA MPV SPECIAL2000, 2001, 2002, 2003

CARS USING HARNESS 24241205
Buick Lesabre 4DR CUSTOM2000, 2001, 2002
Buick Lesabre 4DR LIMITED2000, 2001, 2002
Buick Park Avenue 4DR Base1999, 2000, 2001, 2002
Buick Park Avenue 4DR ULTRA1999, 2000, 2001, 2002
Chevrolet Impala 4DR Base2000, 2001, 2002
Chevrolet Impala 4DR LS2000, 2001, 2002
Chevrolet Monte Carlo LS2000, 2001, 2002
Chevrolet Monte Carlo SS2000, 2001, 2002
Oldsmobile Aurora 4DR 3.5L2001, 2002
Oldsmobile Aurora 4DR Base2001, 2002
Pontiac Bonneville 4DR SE2000, 2001, 2002
Pontiac Bonneville 4DR SLE SSEI2000, 2001, 2002
Pontiac Bonneville 4DR SSEI2000, 2001, 2002

CARS USING HARNESS 24237264
Buick Century 4DR Custom2003, 2004, 2005
Buick Century 4DR LIMITED2003
Buick LaCrosse 4DR Base2005, 2006, 2007, 2008, 2009
Buick LaCrosse 4DR CX2005, 2006, 2007, 2008, 2009
Buick LaCrosse 4DR CXL2005, 2006, 2007, 2008, 2009
Buick LaCrosse 4DR CXS2005, 2006, 2007, 2008, 2009
Buick LaCrosse 4DR SS2008, 2009
Buick Lesabre 4DR CUSTOM2003, 2004, 2005
Buick Lesabre 4DR LIMITED2003, 2004, 2005
Buick Lucerne 4DR CX2006, 2007, 2008, 2009, 2010, 2011
Buick Lucerne 4DR CX CLOTH2009, 2010, 2011
Buick Lucerne 4DR CX CLOTH PLUS2009, 2010, 2011
Buick Lucerne 4DR CXL2006, 2007, 2008, 2009, 2010, 2011
Buick Lucerne 4DR CXL LEATHER2009, 2010, 2011
Buick Lucerne 4DR CXL LUX & CONFIDENCE2009, 2010, 2011
Buick Lucerne 4DR CXL LUXURY2009, 2010, 2011
Buick Lucerne 4DR CXL SPECIAL EDITION2006, 2007, 2008, 2009, 2010, 2011
Buick Lucerne 4DR CXL ULTRA2009, 2010, 2011
Buick Park Avenue 4DR Base2003, 2004, 2005
Buick Park Avenue 4DR ULTRA2003, 2004, 2005
Buick Regal 4DR GS2003, 2004
Buick Regal 4DR LS2003, 2004
Buick RENDEVOUS 4DR2003, 2004, 2005, 2006, 2007
Buick Rendezvous2003, 2004, 2005, 2006, 2007
Buick TERRAZA2005, 2006, 2007
Chevrolet Impala 4DR 50TH ANN SPL2008
Chevrolet Impala 4DR Base2003, 2004, 2005
Chevrolet Impala 4DR FLEET 1FL2011
Chevrolet Impala 4DR FLEET 2FL2011
Chevrolet Impala 4DR LS2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
Chevrolet Impala 4DR LT2006, 2007, 2008, 2009, 2010
Chevrolet Impala 4DR LT 3.52006, 2007, 2008, 2009, 2010, 2011
Chevrolet Impala 4DR LTZ2006, 2007, 2008, 2009, 2010, 2011
Chevrolet Impala 4DR POLICE2004, 2006, 2007, 2008, 2009, 2010, 2011
Chevrolet Impala 4DR SS2004, 2005, 2006, 2007, 2008, 2009
Chevrolet Impala 4DR TAXI2011
Chevrolet Malibu 4DR SS2006, 2007
Chevrolet Malibu Hatchback MAXX SS2006, 2007
Chevrolet Monte Carlo Base2003, 2004, 2005
Chevrolet Monte Carlo LS2003, 2004, 2005, 2006, 2007
Chevrolet Monte Carlo LS LS2003, 2004, 2005
Chevrolet Monte Carlo LT2005, 2006, 2007
Chevrolet Monte Carlo LT 3.52006
Chevrolet Monte Carlo LTZ2006
Chevrolet Monte Carlo SS2003, 2004, 2006, 2007
Chevrolet Monte Carlo SS HI Sport2004, 2005
Chevrolet UPLANDER2005, 2006, 2007, 2008, 2009
Chevrolet Venture2003, 2004, 2005
Oldsmobile Silhouette2003, 2004
Pontiac Aztek Hatchback Base2003, 2004, 2005
Pontiac Aztek Hatchback GT2003, 2004, 2005
Pontiac Bonneville 4DR SE2003, 2004, 2005
Pontiac Bonneville 4DR SLE2005
Pontiac Bonneville 4DR SLE GXP2004, 2005
Pontiac Bonneville 4DR SLE SSEI2003
Pontiac Bonneville 4DR SSEI2003
Pontiac G6 Base2006, 2009
Pontiac G6 GT2007, 2008, 2009f
Pontiac G6 GT UPLEVEL2009
Pontiac G6 GXP2006
Pontiac Grand Prix 4DR Base2006, 2007, 2008
Pontiac Grand Prix 4DR GT2003, 2004, 2005, 2006, 2007
Pontiac Grand Prix 4DR GT22004, 2005
Pontiac Grand Prix 4DR GTP2003, 2004, 2005, 2006
Pontiac Grand Prix 4DR GXP2005, 2006, 2007, 2008
Pontiac Grand Prix 4DR SE2003
Pontiac Grand Prix 4DR SE12003
Pontiac MONTANA EXT MOBILITY SPECIAL2003, 2004, 2005
Pontiac MONTANA MPV BASE2003, 2004, 2005
Pontiac MONTANA MPV EXT2003, 2004, 2005
Pontiac MONTANA MPV EXT BASE2003, 2004, 2005
Pontiac MONTANA MPV EXT UPLEVEL2003, 2004, 2005
Pontiac MONTANA MPV SPECIAL2003, 2004, 2005
Pontiac SV-62005, 2006, 2007, 2008, 2009
Saturn Relay2005, 2006, 2007
GM FWD Transmission ID chart

2001 doesn't have a 3.9. Doesn't matter. Replace the transmission or rebuild it. Don't waste time and money to have certain parts fixed.

That code is for the transmission. It indicates a mechanical failure within the transmission. There is a technical service bulletin issued by GM in regards to it. TSB# 01-07-30-036C. If the car is under warranty GM will fix it. If not, cost of repair will has to be paid by you. This issue is related to 2nd and 3rd gears, could has debris in orifice #29 inside transmission.

The linkage to the trans may be bent

Needs a converter inside the transmission, take it to transmission shop and have them give you a free estimate

If you need further help, I’m available over the phone at https://www.6ya.com/expert/michael_2094e122cc46f24c

It might as simple as a loose shift lever at the transmission or it could actualy be the transmission itself (splipping clutch pack). I would have the transmission checked and serviced if you haven't done so already.

Usually valve body is stuck,flush?did they change trans fluid and filter if not recommended you change filter and a bottle trans x it helped a 2001 impala ls I had transmission was good till the day sold it

so it idles fine as long as it is in park. when you shift it into gear it stalls?

Does it feel like if you had a manual transmission and you let out the clutch with your foot on the brake?

if you are in a place you can do this safely without hitting anything try starting it while the transmission is in neutral. After it starts put it into either drive or reverse with your foot off the brake. Does it start to move, then the rpms start to drop fast and then stall?

Again if you can do this in a safe place what happens if you do the above and as soon as it goes into gear you give it a little gas? Will it move and stay running until you come to a stop? If yes the torque converter is locked.

Unfortunately from the description of the fluid and this symptom your likely looking at replacing the transmission.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa