Hi Bob,

I usually stick the negative probe into that braided wire between the cam cover and the firewall for a ground reference. It makes better contact than using a clip on the strut mount stud.

Measuring the O2 sensor heater resistance cold yields 3.0 ohms. Running the engine for one minute and shutting down, quickly measuring resistance of the sensor heater yields 5.0 ohms.

This bothers me. It sounds more like a little warmth from the exhaust. A minute should be enough to get that sensor warmed fully by electricity. Check at that red/yellow lead (backprobe while it is running) to be sure you have battery voltage to the heater while it is connected. It is supplied through fuse 4. So much of this depends on how quickly you get the meter on the white leads after cutting power.

...when measuring voltage from the O2 output (green wire connected to the sensor) to the low side of the heater (black wire) ...

Yes, I expect the low side of the heater is indeed above ground coincidentally by about the same amount you see on the sensor output when it is pegged lean. Just to be sure (I suppose you're wondering if the two are shorted) check the sensor output key on motor not running and sensor cold. Should see 550 mV from the ECU. Disconnect to prove to yourself it is coming from the ECU and then, if the sensor is cold, should not load this down at all.

Most of our readers would just say "put in a new sensor," but they know I like to obtain a conviction first. I wish it were that easy with the expensive AMM.
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Art Benstein near Baltimore

Never, under any circumstances, take a sleeping pill and a laxative on the same night.