I've dealt with this code before on another Sienna. Changed the 2 sensors and coolant lines and that one was fixed. This time, though, I am working on an engine that I installed a year ago. 0 mile rebuild with all new sensors, coil packs, injectors, and a bunch of new stuff...At 1 year and 10K miles I could not believe the sensor was bad, and I dreaded tearing the intake off. I have had other issues with this rebuild. Front engine seals leaked. They were the factory installs. I just recently finished replacing those. Also had a trany converter seal leak, which was a new seal I installeda year ago, so anything is possible. I decided to test a bit more on this. I managed to borrow a borescope and look at the sensors under the intake. A loose sensor will report as bad. I did instal with locktite and torqued to spec a year ago, but again, anything is possible. Others have also complained about the small 2 sensor to 1 plug harness under there. But that all looked good. Took the plug apart just outside the cavity going towards the firewall, not a lot of room there. Sensor side read opens, no shorts. Cable to ECM side blacks were grounded and one white read 3.9kohms. Unplug the E12 plug from ECM and measured the ECM, both 27 and 28 had 3.9kohms. I then grounded 1 end of the wire harness to ring out the cable run and sure enough one white wire was open. I found the connector up near the intal had a broken wire at the pin. This is the only connector on the engine that does not secure the wires within the connector. They can wiggle around and eventually break. I had a an old harness laying around from a used engine I bought for my other van. Cut the connector off at about 4 inches, cut the broken connector off and soldered it up. I also filled the back of the connector with RTV in hopes that would help keep the wires secured. Good luck!