Installation of a locking gas cap really shouldn't require a How-To article.  However, the existing cap on the Impala is attached to the car with a short lanyard.  This prevents loss of the cap and hangs it out of the way during fueling.  I liked this setup but did not think I could retain it if I replaced the cap.

Initially, I was looking for a Stant gas cap.  I have one on my Camaro and that cap has lasted 17 years.  I went to the local Checkers and asked for the cap.  As soon as I said "2001 Impala," the worker cut me off and said "go to the dealer."  I was taken aback and my snappy response was, "Huh?"

She told me that they don't have caps for my car and I would have to go to the dealer.  I had my car for eight months and I was pretty sure that the '00 cap would fit.  This was my second visit to Checkers (this is a new store in Hawaii) and I have not been impressed with their customer service.  She didn't even attempt to research it.

I went home and found the Stant site online.  I e-mailed customer service and got a response the next day.  The Stant part number is 10506 or 11506 - same cap different packaging, so either cap would work fine on this vehicle.  The cap will also fit on the 2000 model.  I went back to Checkers (I know, dumb move) but they only had one cap, the package was open, and the key was missing.  I asked if they had another.  No.  When will they get more in stock?  Dunno, when we sell the last one.  This response was after I told them the last one was missing the key.

Dura gave me the part number for the GM cap so I drove down to the dealer and got it.  I was suprised by the price.  I had always thought that aftermarket accessories would always be better/cheaper than what the dealer offers.  The GM cap was $11.50 and the Stant cap was $14.99.  In addition, as Dura pointed out, you can remove the lanyard from the stock cap and attach it to the locking GM one.  And that's what this article will look at.

Dura said this was on the box he got:
Gas Cap L, Gr. 03.028, 1 #998973, 4580C7

However, this is what was on the box I got:
CAP, Gr. 03.028, 75J0 1 #GT227, 1 #22616294

Best bet is to go to the dealer and ask for the cap that will fit your car.  Note, the cap will fit on either the '00 or '01 so if they tell you they don't have a cap for the '01, ask for the '00.

Sidney just told me that he went to his dealer armed with the part number and the cap he got was a non-locking one.  It could be that someone put the wrong cap in the box, the part number changed, or they discontinued it.  If you want a GM cap, ask your dealer to search their database.

Jeremy picked up a locking cap for his '03 Impala and got:
CAP, Gr. 03.028, 75A3 1 #GT227, 1 #22660006

This is what you get from the dealer.  You probably can't see it too well in the photo, but the plastic bag that held the cap and key has a little "Made in Canada" label on it.
Here is a closeup shot of the cap.  The shiny ring that is recessed into the edge of the cap is where the lanyard is attached.
This is the existing cap
The lanyard it attached to the cap via a ring.  This allows the cap to spin freely.
If you hold the cap in the palm of your hand and squeeze the ring from three sides (my thumb, three fingers and palm) a little slack is built up on the fourth side (where the lanyard is).  There is a little gap in the ring and that is where it will pop off the cap.
This is a close up of the gap.
As you apply pressure, lift up on one side of the gap of the ring.  The plan is to lift up one corner of this gap onto the edge of the cap.  When this happens, you can work the ring around the edge of the cap until you free the ring.  It is kind of difficult to describe this motion, It's not like opening Tupperware, where you just lift the cover straight up.  You want to twist the ring.  Watch out, because the ring isn't really flexable and at a certain point it will pop off and you might drop the cap.
There is a top and bottom to this ring.  You can tell by the way the lanyard is attached.  In this photo, this is the side you would push the cap back in to.  So when you install the cap onto the car, this is the side of the ring you would see.  Reattach the locking cap just like you removed the old one. Place one corner of the gap in the ring into the channel in the cap and work your way around.  It will snap right in.
The only real thing I dislike about this cap is you need two hands to lock/unlock it.  When the cap is unlocked, it works exactly like the stock cap.  When you lock it, the top spins freely.  However, because the top spins, when you put the key in, you have to hold the cap to turn the key or the cap will twist with the key.