the seedometer, gas, temp, are all one cluster, you have to remove the dash, after that it is held in place by 4 or 5 9/32 hex head screws. unscrew the unit and unplug, the plug should be on the...
the base model impala in 2004 has a 3.4 L V6, while the LS, and SS both have a 3.8 L V6.
It is built into the Mass Air Flow sensor. That is in the air duct hose that comes from the air filter box and goes to the engine.
1 up stream before the converter rear exhaust manifold (closest to fire wall )2 down stream is after the converter.
I have a 2001 Impala with a 3.8L engine. With new oil and tires at full pressure I get 28mpg. 26 prior to service