Remove oxygen sensor with rusted nuts

After trying to remove the front oxygen sensor for some hours, I finally managed to get the rusted nuts off. What helped me after having tried heating them until red, all types of pipes, rust penetrating oils and vise grips etc. with no luck, was a nut splitting tool and a small metal saw. The oxygen sensor was then easily removed. Unfortunately it left me with the threads on one of the studs in quite a bad condition. It was not possible to thread in a new M8 nut. What made the threads work again, were threading in a M8 threading die. This had to be done without the correct tap handle as there are not enough space around the exhaust. It worked by using a set of adjustable pliers. This makes no marks on the threading die as they are usually made of very hard steel, but the adjustable pliers lost some metal in the operation...

A tip on how to test the oxygen sensor is to use a volt meter to measure the voltage between ground and pin ox1 (ox2 for the rear sensor) on the diagnostic connector mounted under the left front seat (at least in my Previa). The motor must be warm, running at 2500 rpm and the measured voltage shall be switching between more than 0.6 Volts and less than 0.2 Volts once per second or so. Unfortunately mine gives still a low voltage, only up to 0.2 Volts after running it over 4000 rpm and back to idle. Seems like it is still running lean...