I found this info online and decided to share with the people who own a 2004 Impala, or car with this type of transmission. Doing this will save you the $3,000 that the transmission shop or your mechanic will try to charge you to Rebuild or replace your transmission when you tell them that your transmission is skipping/delaying or giving you a engine error code.
This Transmission is for the BONE STOCK Impala. not the LS or SS (sorry guys different transmission)

I got the transgo sk4t65e kit and changed accumulator springs and spacers, works great, really easy to do. drop pan, unbolt accumulator housing, pull out piston, replace spring with ones in kit and reinstall. takes about 160 bucks and about an hour of your time.

Step one buy the kit, go onto Ebay and search transgo sk4t65e kit. If you try to buy one of these at your trans/mechanic shops they will not have it or charge you a arm and a leg for it. On Ebay it cost Around or less than $60.

Go and Get some transmission fluid and a trans filter since you are going to removing all of the old fluid might as well do some preventative maintenance on your car as well as fixing a problem. I also put a drain plug in mine to make changing the fluid easy the next time

Step 1: How to Repair Skipping/missing Shift for 04 Impala Transmission

This is what the kit will look like. For this instructable you will IGNORE the directions of removing the transmission, we are only going to use the parts that are intended for the First/Second and Second/Third module which is on the BOTTOM of the transmission.

Step 2: How to Repair Skipping/missing Shift for 04 Impala Transmission

Now that you have all the materials

1. drain tranny fluid
2. take off tranny pan , keep the gasket because it is more than likely going to be the reusable type ( metal lined)
3. Unbolt accumulator housing, there are 4 or 5 bolts that mount the accumulator to the tranny, but all of the bolts for the accumulator need to come out sooner or later.
4. It will have 3 lines going to the accumulator housing, they just slip out with a little finesse
5. REMEMBER THE ORDER THAT THE COPPER HOSES ARE IN!! IF YOU CANT TAKE A PIC WITH YOUR CELLPHONE FOR REFF OR ELSE YOU WILL KILL YOUR TRANSMISSION!!!!
6. The bolts are METRIC (6 or 8 for the transmission, and 6 i think for the module)

Step 3: How to Repair Skipping/missing Shift for 04 Impala Transmission

Here is the directions for the section that we are going to work on.

You are going to use the springs and shims, and gaskets if yours are worn out

Step 4: How to Repair Skipping/missing Shift for 04 Impala Transmission

4T65E accumulator removed and disassembled showing 1-2, 2-3 pistons/springs. Now

1. Open up accumulator housing and you will see a gasket, which can be reused, and 2 pistons
2. Pull those out along with the springs, you will reuse the springs on top of the piston, but not below the piston
3. The transgo kit uses 3 springs in the 3rd position along with a shim above and below the piston (It will tell you this in the kit, The springs are color coated and it tells you which colored springs go where so unless you are colorblind you cant mess this part up)
4. 2nd uses 2 springs with a shim below the piston.
5. The rods in which the pistons ride on will fit into the bore at the bottom of the accumulator housing
6. Clean gaskets and bolt everything back together, make sure the 3 lines are secure and fully installed into the housing
7. 8mm bolts - 8 ft lbs(module), 10mm bolts- 18 ft lbs(transmission, I did 12ft lbs, remember this is aluminum which is soft and easy to strip, unlike steel)
8. Now pat yourself on the back for saving $2,800 plus from the transmission shops.

One final note. This fixed my problem (transmission skipping and shifting hard into first/second gear) if you are having problems in fourth gear then you will have to remove your transmission and use the rest of the parts in the kit. now the only problem I have is sometimes it shifts too fast into first gear and wants to haul ass. Better that then the other problem.