Truck throws a code 21 and does that have any effect on the way my 91 4x4 pickup runs

 

Code 21 is for your oxygen sensor, it may not effect your running but I bet it's impacting mpg. If your wiring is good the oxygen sensor is probably bad.

 

PO did a lil hacking to harness but I got a used harness and just confused on best way to go about putting it on

 

You have a full engine/ecu harness? I would get in there with a voltmeter and check connections before replacing the full harness. You may have a bad sensor, no point in replacing wiring if that's the case.What engine do you have? Oxygen sensor testing is pretty straight forward.

 

Harness has to be replaced the po spliced wires for some reason to make the master power switch a flip switch in truck you have to flip that then you can crank it with the key. But how do you test it that way I know it its bad and isn't there 2. O2 sensors

 

This is the procedure for testing.This is from a 93 fsm your specs should be the same. You didn't specify engine so I'll post both. 22re http://personal.utulsa.edu/~nathan-b...42oxygense.pdf 3vze http://personal.utulsa.edu/~nathan-b...01heatedox.pdf

 

I have ohm meter but anything to do with electrical I suck so I'm a little confuse on still how to do it

 

Like what setting do I put ohm meter on and the 2nd page of what link you posted confused me but I have the 22re

 

You want to set your meter to check for resistance (The symbol looks similar to a horseshoe). You first want to check how many times the meter deflects in a ten second period. Needle type meters are best for this but it can be done with a digital meter as well. If if deflects more than 6 times in the ten second period it's normal and the sensor is good. If not you continue to page 2 and disconnect the pcv hose and check the voltage across the specified terminals. I wouldn't worry too much about that portion. You want to focus on the first page, and how many times it fluctuates in the ten second period.

 

If I get time I'm going to do this tomorrow and if I run into trouble I'll post back so you could maybe help me out

 

Where exactly is o2 sensor

 

Just in front of catalytic convertor if you look under the truck near the drivers door you will see it in the exhaust pipe.