2.5L ('98-'01) O2 sensor heating element Issue, 1998 Ranger 2.5L

[yelir]

1998, 2.5L ranger, 2WD, manual

So I have had issues with the upstream O2 sensor since i got the truck over a year and a half ago. I dont exactly remember when it started throwing a code for it, maybe right after I bought it, certainly not long after I bought it. It seemed like it used to get better gas mileage. I went on a road trip and the truck then started bucking and surging and not going over about 30mph, and it was throwing the code P0135 (failure at heating element in O2 sensor). then sometimes I would fire the truck up and it would seem to run fine, and get up to speed. Regardless I had the O2 sensor replaced.
This truck is from Minnesota so the rust had fused the sensor to the exhaust and I, nor any shop in town could get it to budge, so the shop said they would break it off and drill it out and weld a threaded mount to the exhaust. Well... they did all that except instead of tapping or welding threads in they just welded the O2 sensor to the exhaust. I was thinking that may have damaged the sensor because the code came right back, however the bucking and speed issues have never happened since. All of that was over a year ago and the truck has never had those issues again, but it does still throw the code, and a couple other codes (below), and seems to be not getting the best MPG. I'm just getting into looking at the electrical side of things here's what I found.
At the end of the wire where the upstream O2 sensor plugs in I'm getting about -0.3v where i should be getting 12v. Key on. this is the wire that supplies power to the heating element in the sensor. So it makes sense to me that this is why its throwing a code, but I cant seems to find where its losing the voltage, and why its negative!?. It has 12v at the fuse. the ground wire tests fine, so I don't think that is a problem. and I also tested the resistance of the actual sensors heating element and it seems fine, so maybe they didn't damage the sensor with the welding after all.
So my question is, how important is it for the heating element to work. once the exhaust gets hot and heats the sensor it should in theory then be getting the same MPG as without the initial heating right? Im new to all this and I appreciate the help and the people who keep this site rollin. thanks in advance for any insight.

the other codes its throwing are
p1506- engine idle out of factory spec.
p0500- error in speed info recieved from ABS GEM or CTM (my spedo/odometer has never worked and im guessing this has to do with that)

CHeers!

 

[Dirtman]

Well... they did all that except instead of tapping or welding threads in they just welded the O2 sensor to the exhaust

Good god never go back to that shop...

 

[yelir]

Any Ideas???

 

[scotts90ranger]

If they welded the sensor in, the sensor is likely broken. the sensor part is a 0-1V signal where it makes voltage when working right, the heater element is ceramic and fragile. O2 sensors are rated to be dropped on concrete from 3' but it is NOT recommended, so the heat of welding probably wasn't good for it...

I'm not sure how the heater circuit works on Rangers of that time frame, there's a few semi standard ways to do it: get constant key on power and the computer switches the ground on when it needs it, constant ground and switched power when it is needed.