I replaced my main (upstream) O2 sensors today. I know they were replaced back around 50k miles, but I have no record of whether they were done over the past 140k, so I figured as long as I'm going to the trouble to pull them, I might as well have new ones on hand ready to install. I didn't have any CELs for them, but then again, it is OBDI.
Getting them out was tricky but not impossible. I used one of the offset O2 sensor sockets from Harbor Freight. That worked great on the driver's side, but required a wobble attachment on the passenger side to get securely on the sensor's base.
The hardest part was probably disconnecting (and reconnecting, on one side) the plugs. I was following the lexls
DIY and the orientation seemed a bit different... I finally figured out the release tabs are on the top and you press down on the back portion to release the clip up front. I also had to straighten the pins to get one of the new ones to line up properly and clip back in.
Anyway, I measured the resistance of the old ones I took off, and they were 6.6 and 6.7 ohms at 74 degrees. The DIY and the service manual (specifically for the downstream sensors; didn't see anything about upstream) both say 5.1 to 6.3 ohms at 68 degrees, but no sliding scale with temperature changes like most sensors have. I would think the difference between 68 and 74 degrees is negligible as far as anything exhaust related is concerned, so is it safe to say my old sensors were out of range and a bit "lazy?"
I forgot to check the resistance of the new ones at the same temp's before plugging them in...
Interestingly, the old ones are labeled as Toyota 89465-24050 ND 065500-3491 4T and have a significantly shorter wire harness than the new Denso #234-4211 sensors. I'm guessing that means the old ones were factory replacements back in '98 at 50k. Shortly after that, the car disappears from dealer service records, making it unlikely a private mechanic replaced them in the interim.