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There is an open circuit in the A/F heater circuit. I suspect that the problem is on the power side of the heater circuit, check if there is 12 volts going to the A/F sensors on the heater circuit. Check if there is 12 volts to and from the heater relay.
I concur with the advice that @Nevada_545 stated about the issue. You stated that at least two different techs have already gone over the wiring and have missed on finding the real problem. Perhaps the issue is due to a high resistance connection to the heaters and voltage checks were done while no real load was on the circuit. This would fool a unsuspecting tech to think that the voltage supply was good and there was no issue there, but there really is. The voltage level to the load would drop when there is a current load on the circuit. However the power to the sensor heaters is delivered to them that wiring needs to be checked out. Preferably by checking the resistance of the wires from the power source to the heaters. There also seems to be a relay involved so the operation of that relay circuit needs to be checked out. It is most likely controlled via the ECU.
Why is the discussion “hidden” . . . ?
I clicked on it and read the whole thing. I don’t see any foul language or anything else objectionable . . . ?