I am finally thinking of upgrading to a 2012/13 S4 from my 01 A4. The cars that I am looking at have about 45 k miles. Are there any known problems with these cars?? Any particular trim I should be looking for?

 

Do it. I just picked up a 2014 S4 and love it. Mine is a Premium Plus with Sport Differential, S-tronic, B&O sound, two-tone Nappa Leather. Just love it. I think the Sport Differential and B&O sound are my top two options I wanted.

 

A little late to the part buy I would have to agree. The s4 is a beast (for the asking price).
I also went with the B&O setup, sport differential and Napa leather (+ not so important extras). However I wanted a manual so it took me 6 months to find mine and I'm happy

As for issues - there aren't many. Mainly:
1. Waterpump - early versions hat an issue with water-pumps (plastic impellers) but they should all have been replaced by now with metal impellers.
2. Thermostat is a little weak and is known to quit working around 60k.
3. Crank pulley is a little weak. There have been reports of the pulley failing - mostly after people went stage 2 though.
4. Squeaky brakes. The OEM brakes have some good stopping power but they squeak when the temps go down. Brake dust is also excessive for my taste - however this can all be easily corrected on the first DIY brake job with better pads and rotors.

Overall I've heard very good things about this car/engine - 1.5 years and 15k later I would have to agree this car has been rock solid and fun!