Code P0175 96' C1500

[SoCalSlaughter]

I recently bought a 1996 Chevy C1500 5.7. Its pretty clean, all stock. Runs and drives fine. I have to smog the truck soon in Cali, and the computer keeps giving me a P0175. I decided to give the truck a tune up, all new O2 sensors, spark plugs and wires, dizy cap and rotor and new air filter. Reset the computer and take the truck for a drive, and after about driving for 15 minutes, the CEL comes on again with the same code. Did a Live data read of all the O2 sensors and they are all operating correctly. The truck runs fine, but it wont pass smog if the light is on. Any ideas on what i can check on? test? replace? Thanks guys!!