Low voltage in o2 sensors on a 97 gmc
 

One of my friends went and replaced all 4 o2 sensors on his truck, and now hes got all 4 codes popping up
All the connections are fine, and we tested the voltage at the connector and it was fine
I dont get where it would be bad now, but he cant hardly squeal a tire anymore because of it, and hes only getting like 9 mpgs compared to the 14 before
And this is actually a 97 sonoma highrider

did u use ac delco parts. and did u reset the pcm. for the new o2`s ?

that we didnt do, and im not sure on the delco part because they asked after it was done
Dont you just unhook the battery to reset the pcm?

What DTC's are you getting, and what order were they retrieved?

There's no need to reset the PCM after replacing O2 sensors unless you want to reset Long Term Fuel Trim, (it will readjust on its own over time). To reset LTFT you must use a capable scan tool. The el cheapo $300 or $400 scan tools that the auto parts stores use will not reset LTFT. Disconnecting the battery will not reset the PCM: It will only turn the SES light off, and it will remain off only if there are no hard faults present. It will not clear any codes, reset monitors, or reset fuel trim.