My check engine light came on last week on my way to an appointment. I have a 2000 Tundra V8 4WD with 137,000 miles on it. So that night after work I stopped at Walmart and bought a OBD2 Scanner for $104 with tax. I got home and hooked it up and found it was an O2 Sensor. The one behind the cat on the right side. So I got on line to find on and everyone priced the one on the left side because they didn't bother to read it on the screen which on was which since the truck has 4 of them. I finally found it at OReilly's for $ 111 with tax. I sprayed the stud with PB Blaster penetrating oil and let it set for about 20 minutes. The top nut came off pretty easy but the bottom nut I had to drive a 11mm socket on the nut to get it off because where the nut rust the 12mm socket was too big. I changed the sensor out put some anti seize on the studs and two new nuts. I then cleared the codes out and drove it around the block until the engine was good and warmed up and the codes didn't come back on. For about $215 dollars I (hopefully) fixed my truck and the next time the check engine light comes on I will be able to fix my truck cheaper than taking it to the dealer. Here is the website for the OBD2 Scanner model 3100C . If anyone wants to look at it. It's priced higher on the site than what I payed for it and it can be hooked up to my computer for updates. C A N O B D 2 . C O M Hope I wasn't to long winded on my first post.