A buddy of mine went through two cut and solder O2 sensors on for his '88 efi runner. He took back the first one thinking it was bad. After the second one was still triggering a code, he asked one of the techs at his store about the problem. The tech told him to make sure the wires are the same length, which he new the weren't. After work he shortend the wires and the sensor worked perfectly.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed