"i am testing the functionality of my o2 in 91 740wgn non turbo.according to the process in the brickboard .when key is set to start position (engine off)it should read battery voltage at heater power wire(red).i don't see any voltage drop until the car is running."

• The heater voltage is applied from.by the Fuel Relay. Except for the brief pump run at Key On, the Fuel relay os only energized during cranking and running. So what you see on the heater wires is normal.

But I am interested (read confused) when you say:
"...it does the same in my 93 940 sedan.also when probing the signal wire(white) and sig return(green) the voltage fluctuate
between .01 and 1.1V..."

• In my experience here, the non-turbo '91 740 (and '93 940 like mine) would have the Regina FI system. That means a unique (non-Bosch) O2 sensor would be required. That Titania-based sensor has no green wire at all, and has the following wire (color) assignments:

Black Connection: Signal out ("return")
Red Heat + (positive)
White Heat - (negative) and Signal feed

Till now I know of only one brand of Titania O2 sensor, the NTK at Sparkplugs.com .

Could you give us more info on your O2 sensor brand/type, where purchased etc.



--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.