Box end, assuming the metal hex on the sensor is still solid metal and not badly rusted away. As long as the 12-point box has something solid to grab, you should be okay.
- If you need more leverage (if you have room for it), you can slip a pipe over the open end of the wrench.
- Or use this trick: https://youtu.be/SDI4j1LgqV4?t=35 Be careful about the wrenches disengaging and pinching your hand though. In the comments, someone suggested slipping the box end of a much larger wrench over the open end of the smaller wrench, which is less prone to slip and thus safer.