I had to join this forum to express my thanks, for the advice on the TCA to fix my ac. After finding no leaks in my system, I came across this forum and found the solution. I just bought a 2001 Frontier XE 2wd recently, and the previous owner said he thought it just needed freon added. It turned out to be the TCA, which I learned about on this site. It also had a service engine soon light on, which he said autozone told him was for a catalytic converter. I was able to get him to knock 400 bucks off the price because of that. I took it to my local az, and they said the code was for an o2 sensor. He advised me to check my fuel filter and check for vacum and exhaust leaks because that could give off a false o2 code. The fuel filter looked original (the truck only has 99000 mi) and sure enough as soon as I replaced it, the truck started running smoother and within 100 - 150 miles, the cel went off. So kudos to him for the advice, and not taking the opportunity to sell me parts I didnt need. Well I just wanted to say hi and I am looking forward to visiting this site on a regular basis to learn even more about my Frontier. Thanks everyone and keep posting.