a month ago i replaced the 02 sensors on my 02 5.9. (p0031 and p0037) reset the PCM and the chek engine whent away for 5 days but than came back?? any ideas??

 

If you still have the old sensors I'd do an Ohm reading on them. See if they were even the problem to begin with before you start having sensors warrantied and costing yourself more time and labor.
I'd check your wires and make sure they aren't frayed or melted. Those codes are for the heated part and if that wire is chewed up, you'll keep throwing codes.

 

thanks! ill check those wires tomorrow after work.

 

Also, what were the codes? have you checked for exhaust leaks? Have you checked the exh manifold bolts?

 

Sounds like a short in harness or possible a leak in the system. Do you have a Federal (2 sensor) or California (4 sensor) Emission Package? Also could you have possibly contaminated the new O2 sensors?

CAUTION: Never apply any type of dielectric grease to the O2S electrical connector, or attempt any soldering of the sensor wiring harness.

CAUTION: Also do not add anti-seize compound to the threads of the O2S, the factory has already used a special coating on them.