Ok, so this forum was so helpful and knowledgeable with the installation of my power seats I thought I'd ask for some advice here; So here goes, a little background; I own a 2002 Silverado 1500 LS Ext Cab 4.8L w/ 146K miles. Now let me start off by saying, for the past 10 years I have not had any problems with her AT all. Other than a new oil, fuel filter, and air filter I've never had her serviced/tuned up - which i'm sure may now be the culprit of my problems. Approximately 6-7 months ago, I received my very first SES light. I went out bought a bluetooth OBDII reader and purchased "Torque" for my Android phone. After reading the code (can't recall the exact code at this moment), I googled the error and learned my O2 Sensor on the driers side, after the Cat (If I recall correctly) was bad - or so the error said. Ordered a new sensor from Amazon, replaced it, and everything seemed peachy. About 3 months after, I noticed some oil leaking from the rear differential however paid little attention... Fast forward to roughly 3-4 weeks ago, I'm driving home from work and while doing about 70 mph on the freeway, my truck begins to surge. I gave it a some more gas however it continually surged and acted as if it was going to die out. No SES light, just the surging problems. Was able to drive her home, parked it, let her rest and started her back up a few days later. No rough idling or knocking, and at low speeds everything was fine. It seemed when I would go above 25mph or uphill the truck surged again, identical to what was felt on the freeway - Still no SES light. For kicks, I hooked up my OBD reader and scanned for errors and received P0171/P0174 errors. Found out these were "lean" errors and that there could be a series of potential fixes/problems. I started off with the basics; replaced fuel filter and added new o-rings, cleaned MAF, and added some fuel cleaner. Truck ran like a champ the first 60 miles however thereafter, same surging. Took it to a mechanic (I know nothing about cars) explained my problems and they advised it was most likely the fuel pump - apparently these have a 10 year lifespan which I was/am at. While I was there I told the mechanic to Tune her up - they suggested new plugs/wires, considering I had never done it in 10 years. I also asked to check out the rear differential leak. Phone call later that day, I was told my drive shaft was the cause of the leak, new barrings needed to be installed and the back end rebuilt. $2200 (Rear diff/drive shaft rebuild, tune up, fuel pump, new rotors on rear + e-break, and alignment) Pick the truck up last week and everything seems peachy. So today I'm driving along and out of nowhere my SES light goes off. I read the error and it throws out a P0420... The truck is still driving great but I'm now concerned, is this gonna be another "costly" repair. I've already arranged for the mechanic who fixed it the first time to check it out, but I wanna get an idea of what ya'll think with everything I've just laid out. So my questions are; Is the P0420 (defective cat) coincidence, or is it defective a result of one of these above items being serviced/repaired. I'm assuming the mechanic is gonna tell me I have to now replace the CAT, is there anything I should do/try before hand... Also, what do you think about the cost of repairs I listed above, seem reasonable?