Has anybody thought to look for wires laying on exhaust parts? That is the usual cause of o2 sensor "failure", lazy mechanic again.

All of that "bad exhaust pipe" and "cracked manifold"; I am assuming what really happened was a cracked flex pipe?

What was the reason given for replacing the o2 sensors? Lazy mechanics love to blame them.

I would be looking for a new "facility".

Anyway, you have a wiring problem. Varmint eating wires, wires laying on hot thing, wires chaffed, connectors loose etc. Normally they are slap on forehead moments.