Maybe the conntector thats actully hooked into the sensor? Probe it for continuity from the Ecu pin to the sensor and see if u got a solid wire or if theres a break... Have u done a leak down? Or compression test to see if u have a valve issue? Maybe spark plug arching? Like a broke porsolin sorry not the best speller lol.. Umm if its one cylinder maybe ur harness could have a break and causeing it to ground out or something
I vote harness issue!!!!