The following pages might help someone understand how O2 sensors work and why a multimeter or the ECU cannot always show a fault.
https://www.mbs.id.a.../O2_sensors.htm
https://www.mbs.id.a...sensor_test.htm
If you browse around that guy's site and YouTube channel, you may find other information of use. He is a mechanic who did proper diagnostics back in the 70s onwards. He seems to be who got the vehicles that nobody else could fix.
