Replaced the driver's side upstream O2 sensor with a new Denso sensor on the 2003 STR, Cleared codes. CE light was off for a few days now it's back again, same P1647 code! I'm thinking it's a defective... or maybe returned and rewrapped sensor???

 

Or a wiring/connector fault.

 

With O2 sensors I swap them side to side first. If the code follows then swap the sensor out.

Did you swap the correct side? You are correct P1647 is the LH upstream sensor so it sounds like you did.

That code is for a heater failure in the O2 sensor itself. It's function is to heat the O2 sensor at startup until the exhaust gas can do it. So the code should come on immediately at a cold start. Is that when it happens?
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Yes the yellow check engine light comes on right away.
I may have a defective sensor or perhaps the heater fuse is shot?
Just wondering if anyone replaced an O2 sensor and then had it go bad right away.

 

No heater fuse that I know of? But that would be good to check.

The PCM adjusts the heater current from a high of around 6 amps and ramps it down quickly as the exhaust heats the O2 sensor. The heater circuit is only used for just a short time at startup.

Have you measured the resistance between wires on the O2 sensor?

I had to change out one O2 sensor and was able to see the bad one as it measured different from the other side and from the new one I purchased.
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I didn't do this job myself. I had a local shop replace it with a sensor that I bought. Do you know what the resistance should be for a good sensor? I'm thinking of buying another sensor and trying this again.

 

Same as the side that's happy.

There are 2 utterly different types of O2 sensors: the old kind (fitted downstream) and the wideband kind (upstream on 2002.5MY-on cars, not used on original design S-Types). I hope the right kind was used!

 

Used a new Denso 234-9030 for the upstream O2 sensor. I believe there is a O2 sensor heater fuse on the engine compartment.

 

Checked the 30 Amp fuse for the left side O2 heaters in the engine area, it was good.
Cleared the P1647 code and the CEL light went out....for about 10 engine starts and stops. Now the CEL is on again and the same P1647 code!! Ordered a new Denso sensor, I hope it works this time.