I recently purchased an 05 sierra 1500 single cab with 269k miles. It was a fleet truck so i got it really cheap and it was fairly maintained. When i got it a little over a month ago the SES lights was on for low cat efficiency on bank 1, i reset it and it came back within a few days and it's been like that since. Today i checked again and now bank 2 is on also. I'm assuming because of the high mileage they are just plain clogged. Coolant levels have not changed and neither has the engine oil so those two are ruled out as the possible cause. I've decided the best and one of the cheapest routes is to get two Magnaflow 99006HM universal cats and have an exhaust shop cut my old ones and weld the new ones in. Before i do that i'd like to change the pre-cat o2 sensors and here's where the confusion starts. First, i'm either gonna go with Denso or AC Delco sensors. AC delco shows a few different sensors for upstream at significantly different prices. Anyone know the difference between the AC Delco 213-3867 and 2131148? Before having the cats replaced i will replace the front o2's, plugs and wires, air filter, and clean the MAF. Any other suggestions? Also, what is the odds that both rear sensors are bad? I haven't had the truck for long but there doesn't seem to be a loss of power, average fuel economy, and no bad exhaust smell. Should i try cleaning or replacing the rear o2's first? Thanks!