Thanks for the lecture. I am well aware of the merits of pressure connectors. I should have stated my case a little better. All I ever make at work (over 20 years in the electrical industry) are mechanical crimp fittings. Soldered connections will not conduct power very well for very long. No tool is used for crimping unless it is UL listed for the crimp fitting being used in the business I am in. I am also quite familiar with aircraft avionics. The highest start of the art, for electrical fittings on aircraft is a fitting which is crimped, and then soldered to keep moisture and possible corrosion out of the fitting. Beechcraft Aircraft use crimped and soldered electrical fittings. Beechcraft is considered by many to be the best manufactured small aircraft. I don't bother with the soldering on avionics. The low voltage output of the O2 sensor is susceptable to losses at the electrical fittings. Crimping is my preference, but for locations with all the moisture that can splash into an engine compartment, the crimped fitting for the O2 sensor needs to be soldered or sealed against moisture, as you stated. As far as crimped electrical fittings are considered I have encountered a bad fitting on the alternator field connection of a Cessna 172. The factory fitting was intact, appeared good, and would check for continuity with my Fluke meter. However the fitting would not conduct enough current to operate the alternator. Replacing the crimp fitting restored the alternator to operation. I have also had the identical experience on a car. Would you car to explain the work hardening? If you heat copper to cherry red and air cool it, it becomes hardened. I don't think soldering reaches that temperature to harden the copper. If the copper wire breaks off near a solder joint, it is probably because the solder has made the joint ridgid, and the flexing of the wire occurs where the solder ends. The copper will stress and crack there. Yes, if you make the splice with solder, it needs to be in a location that does not flex.