1993 Chevy Van Running Rich trouble code 45

Hi,
A couple months ago, my fuel pump went out. I replaced it and also put in new plugs, plug wires, cap and rotor, fuel filter, air filter, and pcv valve. The van seems to run decently, but the "service engine soon" light came on. By jumping the a and b terminals on the diagnostic terminal, I was able to get trouble codes. They are code 44 - O2 sensor (lean exhaust), and 45 - O2 sensor (rich exhaust). I replace the O2 sensor near the cat converter and cleared the codes. The service engine soon light came on again (it doesn't always stay on, it will go off - intermittent). Code 44 no longer comes up so looks like thats taken care of, however code 45 still comes up and the van has a trottle body so there's no way of adjusting air/fuel mixture. It seems obvious that it must be a sensor. Where should I begin? I don't want to just keep throwing parts at it till its fixed. I was adivsed that the diagnostics is OBD1 so the problem can't be pin pointed and I'm afraid that if I take it to a shop for diagnostics they're going to, like I said, throw parts at it till its fixed, handing me the bill. Thanks so much for any help, Steve