Chevrolet recommends checking the automatic transmission fluid, or oil, in your 2003 Chevy Impala every 7,500 miles (12,000 kilometers) and changing the transmission oil and filter every 100,000 miles (161,000 kilometers) or whenever you detect metal, dirt, moisture or any type of oil contamination. Just like engine fluid, transmission oil wears out over time and becomes filled with foreign particles from internal components. Thus, replacing the fluid at the recommended intervals will help your Impala transmission performance and increase its service life.
Drive or idle your Chevy Impala for about 20 minutes to bring the engine and transmission oil to operating temperature.
Lift the front of your Impala with a floor jack, and place a jack stand on each side under the frame for support. Set the parking brake, and block each of the rear wheels with a chock.
Place a large drain pan under the transmission, and begin to loosen the front and side bolts from the transmission pan gradually until you remove the bolts with a ratchet, long ratchet extension and socket. Loosen the bolts carefully; extremely hot transmission oil will begin to drain out of the pan.
Loosen the rear pan bolts about four turns with the ratchet, long ratchet extension and socket. Carefully pry the pan loose at the front with a plastic scraper to drain as much oil as possible.
Grab a shop rag. Hold the bottom of the drain pan with it and remove the rear pan bolts. Carefully lower the pan over the drain pan, and drain the rest of the transmission oil.
Remove the drain pan, transmission pan and gasket. Pry the transmission gasket loose from underneath the transmission using a long flat-blade screwdriver; remove the gasket and gasket seal.
Clean and wipe clean the transmission and pan mating surfaces with a plastic scraper and a clean, lint-free shop rag. Lightly coat the new transmission gasket with petroleum jelly around its mounting fitting, and install the gasket and new seal. Tap the gasket in place with a rubber mallet.
Replace the transmission pan in its original position along with a new pan gasket, and install the pan mounting bolts finger tight. Tighten the bolts to 10 foot-lbs. with a torque wrench, long ratchet extension and socket. Gradually tighten and torque the bolts using a star pattern. Begin with one of the bolts in the middle section of either side, than go to the opposite side. Tighten the bolts gradually, alternating between the left and right side of the pan, and work your way to each end of the pan until you have properly tightened all the bolts.
Lower your vehicle off the jack stands, and remove the chocks from the rear wheels.
Open the hood and remove the transmission dipstick. Insert a funnel into the dipstick tube and pour 14.8 pints, if you have the 3.4L Chevy engine model, or 14 pints, if you have the 3.8L Chevy engine model, of automatic transmission fluid (ATF) and replace the dipstick. You may use any ATF product designated as Dexron-VI.
Start the engine and let it warm up for about 15 minutes. Shift the transmission through every gear gradually, pausing for two seconds in every gear until you reach the parking gear again. Leave the engine idling.
Pull out the transmission dipstick, wipe it clean with a lint-free shop rag and reinsert the dipstick all the way in. Pull out the dipstick again and check that the oil level reaches the middle of the crosshatch area at the bottom of the dipstick. If you need to add more oil, only add a little bit at a time until it reaches the correct level.
Check for oil leaks around the transmission pan, and turn off the engine.
