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o2 sensors replaced successfully!
A follow-up to my previous post, I replaced all three o2 sensors on our '98 Sienna using the Denso universal sensors. These are the same sensors as the plug-and-play sensors, but at half the cost. Denso provides a nifty butt-splice kit with these sensors, and this plus the fact that the color-coding scheme is the same as the original sensors makes it pretty much a fail-safe replacement. Denso is the OEM part, and at $40 each, it was the obvious choice. Another reason to use the universal sensor is that YOU DON'T HAVE TO WORRY ABOUT HOW TO UNPLUG THE ELECTRICAL CONNECTOR HIDDEN UNDER THE INTAKE PLENUM! What you do with the sensor on the RH side (firewall side) of the engine is get under the van, unscrew the sensor, then standing in front of the vehicle, send a small rope down behind the engine with a large socket tied to it. Then get back under the vehicle, take off the socket and tie the old sensor to the rope. Then get back up, pull the rope up and there you have the old sensor in your hand, still connected to the troublesome plug. Cut the old sensor off, and proceed to use the butt-splice kit to make the electrical connection. Forget about trying to unplug that plug that you can't get both hands on, and you'll also save $33 or more by using the universal sensor instead of the one that already has the plug on it.
My P0420 code is gone and the check engine light is dark. MPG has jumped back up to 19.6 on the last tank (checked after the o2 sensors were changed), and this comes after several consecutive tanks getting 17.5 mpg on average. In fact, the lower MPG was my clue that P0420 was caused by the o2 sensors and not the cat.
In addition to using universal Denso sensors, I would also recommend using PB Blaster and giving it enough time to do its magic! Also, I had very good luck using the o2 socket set borrowed from Advance Auto. (You actually buy them for $29 but then get your money back when returned).
I used to think o2 sensors were the most misdiagnosed part on a car, but in this particular case, and with 260,000 miles on the van, they were ready for a swap-out.