I have not taken a position that there is not a requirement for reference air. I was questioning the design of using "through the wire".

Other designs may still take their reference air from through the wire, but per Walker Products Engineering, they chose a different design.

Question: "For my application, does the Walker Universal O2 sensor (250-23000) draw ambient air as a reference value or not. I am being advised not to solder the wires because they have to breath to allow reference air to get to the inside of the O2 sensor."

Answer: "All zirconia-based oxygen sensors require oxygen at their air reference. Many of the older sensor designs had no provision to "breathe" to the outside air, so they did indeed breathe through the wires, which was very ineffective. Newer sensors, including the Walker 3-wire design in the 250-23000, have a separate path to the outside air through Goretex materials that reject water droplets, but allow air to penetrate. This is much more effective."

As to the discussion of soldering, the Engineer added amplifying information to his earlier email: "In talking to my colleagues, they reminded me that the Walker sensor has a stainless steel wire core surrounded by a multi-stranded copper sheath. Soldering to this wire is difficult, while crimping is easy and effective. Before the crimping, remember to slide on the Walker-supplied shrink tubing, which is lined with a heat-curable epoxy that bonds to the wire insulation and makes a tight seal. It's good stuff."

I appreciated his followup as it was important to our discussion and wanted to share it with all concerned.

Received the Bosch full wire harness replacement today. Observations:
- the two heater wires are quite tightly seated into the body of the sensor. Nothing can pass through there.
- the sensor wire passes into the sensor differently. There appears to be material around the wire as it goes into the body of the sensor. Maybe Bosch uses the same Gortex technique and that is its design.
- about 12" up the wiring harness from the sensor are two zip ties wrapped tightly around outer cover about 4-6" apart. Feeling around it without compromising the cover, it feels like wires have been spliced. Can't tell which method, but my guess is its self sealing crimps. You can see a picture of it at the FCP Groton website.

This has been a good exchange and hopefully folks who go down this path in the future will have more information with which to make a decision.

Cheers!