Oxygen sensor - code 21

[jehagado]

My 92 22RE "check engine" light came on, code 21
Replaced O2 sensor, "check engine" light came on again, code 21
Old O2 sensor has 4 ohms between black wires
New O2 sensor has 10 ohms between black wires (book says 5.1 to 6.3 at 68F)
Ran volt meter test with te1,e1 and vf1, at 2500 rpm voltage fluctuates once per second, normal
Ran volt meter test with vf1 and e1 at 2500 rpm voltage > 0, normal

Need some wisdom, are both my Denso O2 sensors bad ?

 

[OR17TRD]

Did you replace the one before the convertor and after it? Either way if you only replaced one and it still is throwing the code than you either replaced the wrong one or they are both bad. If you still have the old one you took out swap it for the other one and see if it works if not you need two new ones.

 

[jehagado]

Did you replace the one before the convertor and after it?

Just one O2 sensor on 92 trucks, before cat, four wires, two black wires for heater, two colored wires to send .1 to .9 volt to ecm
Seems unlikley old and new O2 sensor are bad, not sure ... someone has seen this ...

 

[OR17TRD]

Some 1989-1995 use 2 sensors, one before the convertor and after it. Check fuses.

 

[jehagado]

I installed another new oxygen sensor, still getting code 21
Get "check engine" immediate turning key on.
Will check continuity on o2 wires (four wires) back to ecm soon, weather almost nice so I can work outside.
My guess is harness problem (loss or continuity) or ecm has failed (if anyone has a guess please post)
Truck ran good all winter with "check engine" and "code 21", however running poorly now, one cylinder drops occasionally, step on gas all four run good, not sure if one problem or two.
Truck is 27 years old with 230k miles, best car/truck I ever had, don't want to let it go.