A/C Switch disables 1 O2 sensor readings, Head scratcher

2002 Nissan Xterra with V6 VG33E.

All 4 new Bosch O2 sensors, I monitor B1 and B2 sensors and when the A/C switch is off there is no response for B1 O2S1 but when I turn on the A/C switch it starts to respond. I even unplugged the A/C compressor but cycling just the switch still causes the weird problem.
I've attached a PDF of the engine wiring diagram.
I'll try and take a video of the problem to aid in diagnosing.

Thanks in advance

Have you back-probed the sensor connector to see what your voltages are doing when switching the A/C on and off?

Actualy today I did backprobe the signal wire with my fluke meter on min/max and it seems the B1 upstream sensor basicalt flatlines after a min or two of switching correctly. I also backprobed the O2 heater circuit and after start it went from battery voltage to about 300 mV so it seems the heater is working.

Thanks again

craigc wrote: Actualy today I did backprobe the signal wire with my fluke meter on min/max and it seems the B1 upstream sensor basicalt flatlines after a min or two of switching correctly.

At what voltage does it flatline? Is the flatline due to the Min/Max limits finally maxing out? What happens if you turn min/max off?

I also backprobed the O2 heater circuit and after start it went from battery voltage to about 300 mV

^^^Hopefully, someone else will chime in on this. This doesn't sound right, to me. 300mv seems a bit high for the ground circuit.^^^

I pulled this from all data:

pole71 wrote: ^^^Hopefully, someone else will chime in on this. This doesn't sound right, to me. 300mv seems a bit high for the ground circuit.^^^[/attachment]

I guess I'm OK with this as long as it's a pulse width modulated heater? :huh: Otherwise that does seem a bit high. Still, I'm not sure a heater circuit fault on an upstream sensor will cause this.

Any fault codes? What are the fuel trims doing when the switch is on and off?