Over the years I have found this thread to be hugely valuable. As a refresher for anyone new here, a useful list of codes can be found on Page 66 of this thread, Post #967. It's a few years old, perhaps there's an updated post somewhere between then and now, but I don't have the patience to go through it.

I'm a Presidential Preferred Member, which is for holders of the Continental Airlines Presidential Plus MasterCard. Recently I was trying to do a one day rental on short notice in Miami, and found that there was no availability for anything but luxury class, with a SmartCar (as if!) available for $48 for the day and a compact class car for $99 a day, and then a limited number of options ranging from $179 for the day and up. Otherwise every other car class was showing as "Sold Out."

Put in the AWD code D005297 - ASAP (ameteur sports assistance program, which I've used many times in the past), no change.

Put in the AWD code J868801 - Florida Visitor Bureau and BOOM! suddenly there was availability for subcompact class right through to full size and beyond. Subcompact, Compact and Intermediate were coming up at $44, $45 and $47 respectively (cheaper than that SmartCar which curiously went up to $50 for the day). Got the same results using D833357 - The American Society of Travel Agents.

Then I put in A074800 - Citicorp Leisure Travel. The rates on the classes above dropped to $32, $33, and $34. I have never used this code before.

I found it very curious that there was no availability for a Preferred Member (or anyone else), but once I input a few AWDs, suddenly there were cars available, at drastically lower prices.

Anyway ... just sharing this experience for any who find it useful, or informative.