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On my 1998 V70 I'm getting a rapidly fluctuating reading of 8-13 volts with my digital voltmeter connected to the green Black wire leading to pin#29. Rear o2 sensor is at constant zero volts.
This wire is part of the heater circuit for the rear o2. I'm wondering if there's a short in the ECM on the heater circuit causing my ECM to send out a Zero voltage signal,,I have swapped out rear o2 sensors so I know its not a sensor problem plus I have the weird voltage at pin#29..Any thoughts please reply, thanks. Sorry no pics for a better visual.
 

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Look at that wire with a scope. It is PWM to vary current to heater circuit to keep it within a temp range.
ECM can read temp via applying the power supply and then decides the pulse width.
Newer full featured electric ranges do a similar thing for more precise heat control that is tailored to the mass of what is being cooked.

Sensors can crack then short. To see if this is the case get engine warmed up and then go drive and on full throttle acceleration the signal should go to .75 or more and on coast after hard accel it will go to .05V or less.
 
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