Originally Posted by btabor
Scott Clark recommended me to wire it to a ground block installed by the battery ground.
I don't know who Scott Clark is, but my initial impression is that he isn't an electrical engineer.
When the engine is running, the alternator is the primary source of electrical power to the car, and thus, the alternator's chassis is the primary ground for the car. Since the alternator is firmly bolted to the engine, we generally accept the engine block and head to be a surrogate "ideal ground" for electrical loads.
The battery only matters when the engine is not running. After it's started, the battery becomes a load, not a source. Running a ground wire all the way back to the battery is pretty close to the worst possible topology. This means that the current has to travel from the source, all the way back to the battery, then through the ground strap to the body, then all the way through the body to the front, then across the ground strap to the engine, and then finally to the alternator.
I mean, you could do worse, but you'd have to really try. (eg: a self-tapping screw loosely driven into an oversized hole in a piece of rusty sheet metal.)
Originally Posted by Schuyler
Why not just do it properly and wire it to the sensor ground at the ECU?
This is the best idea. For the most stable and accurate readings, the ECU and O2 sensor should share the same ground. This is why all of the OBD-II cars run all of their analog sensor grounds back into the ECU itself, whereas the earlier cars grounded them to whatever point was most convenient. What matters most for an analog sensor is not that it has the best possible ground, but that it see the same ground potential as the ECU.