I have a 93 22re P/U truck 2wd with California emissions and according to the engine code I have to replace the primary O2 Sensor. Now to the question. . . there are two one before the cat and one after. . . which is primary and which is secondary? None of the books I have tell me. Anyone know?

 

The O-2 sensor located before the Cat is always the primary.
Depending on where you go. They can cost as much as $400. Dealer, to as little as $95. Universal from parts store. Shop around. I was able to get a direct original equipment replacement for $89. But I had to call around and that took some time.

 

Thank you very much! Always good info here!