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"The O2 sensor is detecting a rich mixture & trying to compensate hence the steady .15 vdc."
I don't think the O2 sensor can compensate for mixture variations. It merely signals what it "sees" to the ECU, as I understand it.
For some reason, the sensor is signaling a very lean mixture. In response, the ECU is going very rich in an effort to compensate for the lean mix indicated by the low O2 voltage.
If you believe the O2 sensor is good, look for an exhaust leak before the sensor, where air can be drawn in between exhaust pulses.
You can also test the sensor (at idle) by disconnecting it from the ECU wire and measuring to see how its output voltage signal responds to "forced" lean and rich conditions.
• Force/fake lean: Ground the green wire going to ECU. Sensor output should go high as ECU goes rich to compensate for what looks like a very lean signal.
• Force/fake Rich: Hold green ECU lead terminal in one hand while putting the other on battery + terminal. [No danger here. The +12vdc potential is harmlessly transmitted to the green lead terminal.] Sensor output should go low as ECU goes lean, and motor may even stall out.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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