Replaced upstream O2 sensor at 85,000
 

Just had my upstream O2 sensor fail causing the annoying check engine light and also to fail emissions testing. Shop wanted $380 to replace the sensor. Actual cost of sensor is approx $80. I did purchase a specific 7/8" (22mm) deep socket ($12) with a cutout to remove and replace the sensor. The best and most likely the only way to to get to the sensor is to open the hood and remove the battery. Looking down thru the hoses, you can see the location of the sensor, but be forewarned...there is VERY little room to maneuver and trying to get proper leverage to remove the sensor is impossible. So...find some on that has a 150 PSI air tank and a good 1/2" air ratchet. That will easily remove the failed sensor. Replacement is very easy. Once I got the battery out and was able to get the air gun in, it took less than 45 seconds to remove and another 45 seconds to install the new. Then we just had to stand around drinking beers and talking shop for 2hrs!

Also, it's a good idea to spray it with penetrating oil first, and let that soak in while the exhaust is warm.

That is correct...PB-Blaster is a great choice for a penetrating spray!