Water pump failure at 140,000 miles, replaced it myself for $50.00 for parts, $28.00 for a special wrench to remove and replace the pump available at O'Reillys autoparts and information from internet.
EGR valve and intake manifold need to clean, will do it myself, (I am not a mechanic, I just learned to fix my own cars by using commonsense).
Transmission speed sensor needs replacement, but it can be fixed by any do it yourselfer. Not a big deal.
Update 14th September 2009:
Learned a lot on how to really look after a Cadillac with minimal effort.
Fuel pump died at approximately 172,000 miles, and ignition lock cylinder had to be replaced around the same time. Considering a total of $1300 to fix the problems, it was worthwhile as it never stopped running thereafter till I traded it in for a cash for clunkers deal, although I felt it could have easily run another 100,000 miles with minimal preventive maintenance and suspension repairs.
Even at 189000 it was giving upto 29 mpg on cruise control set at 65-70 mile per hour.
Did replace a catalytic converter, but I wondered if I could have avoided doing that because a subsequent replacement of spark plugs and wires at 185000 restored the performance, power and mileage to the levels when I initially purchased it at 120K miles.
Air conditioner did need some repairs, however I never took it in for an appropriate diagnostic check.
Rear passenger air strut started leaking around 185,000 miles.
In my opinion these were relatively minor problems considering its age, odometer reading and the number of owners it has had in the past.
I bought this car for my wife mainly because of its gorgeous looks, it looked very majestic with all the hood ornament and Cadillac emblems all over the car, and its gleaming metallic garnet red color. Little did we realize what pleasant surprises we were headed for.
This car gave 26-28 MPG on highway at speeds of 70-80. I noticed initially that it used almost 1 quart of oil every 800 miles or so, which is considered normal for this car. I realized this problem was largely eliminated on changing over to Valvoline 10W30 High Mileage oil while also improving compression noticeably.
This car has received many compliments about its appearance in-spite of its age. With it 300 horse power North-star motor it has surprisingly great acceleration for a big sized sedan along with a silken ride. The load leveling suspension is very dependable. For $6000 and over 26.000 "worthwhile" miles, besides having preserved its aristocratic looks, I have recovered every dime's worth of this car. Every mile hereon is a bonus.
I highly recommend the STS of this vintage to any American young or elderly couple looking for a mid-sized luxury sedan because of their dependability, reliability, scores of features (both technical and non technical) and ridiculously low prices compared to the imported counterparts.
I will love to keep this car as long as it maintains it appearance. My previous car, a 1986 Lincoln Town Car, (still own it because of its classic appeal and mint condition) although equally my favorite cannot be compared with the STS, it's like comparing a rear wheel drive luxury land yacht with a front wheel drive luxury sedan.
The 225R16 60 tires are a wonderful match for this car, for exceptional handling in the rain, I wouldn't recommend hard cornering at high speeds on wet surfaces, although I have never experienced any problem so far.
At nearly 150 K almost everything electrical on this car works perfectly and the climate control mechanisms performs great, except for a louder hum from the blower fan motor when the rear passenger seating area climate control is turned all the way to the maximum. I guess that's not bad for this age and miles.
I want to own the 2 door Eldorado with the same motor and transmission mechanisms for the sporty look and elegant trunk design.
Update 14th September 2009:
True American luxury car built for reliability, speed comfort and actually it is perfectly suited for every age group that is licensed to drive.
The interior are actually made of durable high end quality materials and not the flimsy stuff some other luxury brands offer. This 1996 model had some special features which some other high end luxury models don't offer till today.
The Bose sound system was a dream.
Noticeably there were no leaks on the motor even at 180K miles.
All in all this is one of the sturdiest cars I have owned, considering the abuse it has handled on the highway.