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I believe you'll find that both the NTK Regina and Bosch LH2 O2 sensors are all heated and thus all use 3-wires. Unheated 2-wire sensors date quite far back like into the early 1980s, possibly K-Jet (Art would know, I'm too lazy to check).
The Volvo service green manual wiring diagrams for both Regina and LH2 show the heater circuit on the 2-pin connector plug white wires (the Bosch standard) with one white wire as the ground (doesn't matter which one). The sensor circuit is the black wire with the signal. Ground for the sensor circuit is shown as a coax-type shield on the black wire (becoming a shielded green wire at the firewall), with one end of the shield to the O2 sensor body (exhaust system) and the other end of the shield to chassis ground at the ECU. If there happens to be no shield then presumably for signal ground it's either depending on the exhaust system as chassis ground or else there'd be a 4th wire, which you may find in some universal sensors.
There was considerable argument here once that the sensor got a trickle of air as atmospehric O2 reference through the cable sheath or wire insulation, but that doesn't at all make sense to me and if there's any truth to this then it likely only applied with the earliest generations of unheated sensors.
There's also been considerabe discussion of not using dielectric grease with O2 sensors. It is generally not recommended for the O2 sensor circuit (the heater circuit doesn't matter), not because it might melt and block off atmospheric reference (or trickle down inside the wire to get inside the sensor head as some postulated) or because it's an electrical conductor (which it isn't), but rather it can contain impurities or attract contaminants that might be conductive and have some affect at the millivolt level. Waterproof crimp connections are the preferred choice and Bosch supplies these with many of their universal sensors. I don't know about NTK.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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