575 M manual CEL possible O2 sensor error codes

Technical question on CEL with a 2003 575M Manual with 25K miles. After winter hibernation, battery switch off, battery on tender, turning the battery switch on and going through the learning regimen as described in this blog, engine starts warms up and runs smooth. After street and highway driving of about 40 or 50 miles the CEL comes on. Engine still runs smooth.

1) Using and OBDII scope the error codes were: P1137, P1138 [both are air fuel errors) and P1048 [Cyl 7-8-9 lambda sensor (O2 sensor) malfunction).
2) Erasing the codes, restarting the engine, no CEL, engine runs smooth. With engine running pending code is P1138. 20 or 30 miles later CEL on turns

Questions:
1)) am I seeing the start of old age malfunction of the O2 sensors and should I replace all four, or just the one bank or ???? [FYI the thermocouples were replaced about 5K miles ago when I got erroneous catalytic overheating light. solved the problem]
2) is 02 sensor replacement a relatively simple task or are there associated electrical adjustments better left to a dealers expertise?
3) Suggestions or comments welcome.
Thanks

 

That would be a great find! Unfortunately I do not think it likely, although very definitely worth an inspection, because the car has been primarily garaged in Dallas and Tucson and never sees rain or winter as long as I have owned it (8 years). Thank you for relaying your experience!

 

Thank you VandevanterSH. I started exploring the error codes especially the P1048 code which indicated a possible potential connection issue with O2 sensor. Acting on your post, I pulled apart the electrical connection plug and the internals appeared to be somewhat corroded, so I cleaned them (and the three other front sensor plugs connectors) with the appropriate electrical connection anticorrosion contact cleaner. End of CEL! Sometimes the simplest solution is the correct one. Since the 02 sensors are from 300 to 400 bucks each, I'd say you saved me money, aggravation, and time. Thanks.