They're both a pain in the a** to get to.. BEFORE you run the hell out of your van, taken them both out, and put some "never seize" on the threads, next time you have to remove them, they wont be such a pain in the a** to get out..

I got lucky with both of mine, I just cleaned them with solvent and put them back in and the errors & CEL went out and stayed out.. They dont always need to be replaced, sometimes heavy carbon build up makes them run faulty.

 

This is the best diagram I found, sorry about the crude numbers. But this is for federal emission sensors only.
Anyways here’s the numbers and their info

1: bank 2 sensor 1 upstream
2: bank 1 sensor 1 upstream
3 bank 1 sensor 2 downstream, pre cat
4 bank 2 sensor 2 downstream post cat

 

your p0051 code is low heater voltage on bank 2 sensor 1 (upstream)

Here’s the picture of where the sensors are at, even Cali models.