This evening I replaced an oxygen sensor on the Sequoia that had been keeping the "check engine soon" light on recently. It wasn't that hard to change and I was able to save quite a bit of money by ordering the sensor online. I got it from
Rock Auto - I chose them because they had
good reviews. That saved about $35 over buying it locally at a parts store.

The sensor is a little difficult to reach, but having the truck up on ramps made it much easier. I had to use some WD-40 and a piece of pipe over the wrench handle for additional torque to loosen the old one.

The real trick to changing it was getting this oxygen sensor socket - it fits over the sensor with room for the wire to protrude. I bought it locally at
Advance Auto in Edmond since it was the same price ($11) as online and shipping was more than sales tax.

Overall, it took a little while to get the part and the tool together, but it was well worth it to save a pretty good bit of money if you count the labor costs of having a mechanic change it.
After changing it, I was able to clear the engine error code with my
OBD-II diagnostic tool that I bought a while ago. It was a bit of an investment (not nearly as much as the MSRP listed on that page), but I think it will pay for itself before too long - if not already.