Trouble code 45

My 1994 S-10 is showing trouble code 45 only after the truck is driven on the highway. The "service engine soon" light comes on only after I have driven over 65 MPH for several miles. The light shuts off after I slow to 35 MPH. I then check the computer and Trouble Code 45 comes up. Trouble Code 45 is-Oxygen sensor (O2S) circuit (rich exhaust indicated). Can anyone tell me what my be causing this condition to happen.

Thanks for any help provided.

I'm sorry, this truck is a 1994 with a 4.3 Vortec with CPFI.

send me a pm, or email address, and I'll shoot you the GM
service page to troubleshoot 45

chris
nrt racing