After code reset, have p0440 code and red o2S monitor

I have a 1999 GMC Sonoma 2.2. The exhaust pipe broke off before the converter. I drove it for a while like this and it generated several engine codes (of course it would).

I recently got the front exhaust pipe replaced. Of course the shop unplugged the battery and it reset the computer. About a week later, the p0440 code popped up. It appeared alone with no other codes BUT the O2S monitor is red while the others are green. Just to make sure, I cleared the codes and the same p0440 code and O2S monitor being red came back. I need to get this truck inspected but am not sure how to proceed. I had a mechanic tell me that since there are no error codes for the O2 sensor that they believe it must be working fine. They suggest should concentrate on the EVAP system. But why is a monitor red with no codes? Any ideas? Thanks!

Do you smell fuel vapor around the vehicule? If yes,check your gas cap for tightness and around the rubber gasket for nicks and dry rot. Since this vehicule is 22 year old I would simply replace it.