Obx header O2 sensor

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4 FEET?!
That's WAY too long. The method you used to extend it may have made it worse too... You never want to solder a low voltage/small signal wire. I wouldn't try extending it more than a foot or damn near.

In three feet exhaust can loose a lot of energy. Think about it this way... on a very free flowing exhaust the air coming out of the muffler probably is still under 200?; It leaves the head around 1400?; Average combustion temps are around 1600-1700?.... That's a BIG drop and VERY fast.

Widebands aren't anywhere near as touchy, but I once had to move one because it was overheating in the stock location... we didn't move it more than a foot and it was fine. The first four feet or so are where the exhaust sees the biggest drop in temperature after leaving the cylinder. But, as mentioned, the heat thing isn't the biggest issue just wire in a heated sensor instead.

The nice thing with widebands is that they're full scale and use a much wider voltage range, a minor drop isn't going to affect much; Narrowbands on the other hand use a signal between .3 and .6 volts to declare an optimum mixture... outside of that range the ECU gets no usable information at all.