Well, the truck has been running great since the head rebuild, but now I have another issue.

I noticed a day or two ago that the truck seemed to be surging ever so slightly at constant throttle. So I whipped out the trusty code reader just to see if I was crazy or not, and sure enough I got two codes.

PO 141 O2 heater circuit bank 1 sensor 2
PO136 O2 circuit bank 1 sensor 2

Now, here is my thought on these.....I assume this means the rear O2 sensor is bad. The truck is a '96 ODBII, 4.9L, 218,000 miles, and has three O2 sensors that I can find. One in each manifold, and one in the single pipe behind the Cat.

PO had replaced the other two O2 sensors last year. The third looks original, or way older than last year.

Any thoughts?

As always, thanks for you wisdom.