What fuse was it, in case anyone else has same problem?While I don't want to jinx it, I've done a few hundred kilometers and the CEL seems to be gone.
The problem: A fucking fuse.
After about 8 months of the CEL appearing at random and replacing an o2 sensor, it turns out that my CEL was due to a faulty god damn fuse. The fuse was half melted inside so it would intermittently fail and cause the CEL.
Unbelievable.