[91CAStang]

Hello, I have a 1991 Mustang - 4 cyl 2.3L DIS

We are nearing the need for a California smog check, but are experiencing an intermittent CEL condition. This has persisted for several months. I did both a KOEO and a KOER test, sending the pulses to the CEL.

For the EO test, I seem to be getting three "1" flashes, followed by a very long pause, then three "1" flashes again, another very long pause, then a "1", long pause, then "172".

For the ER test, I get an immediate "2", then the idle fluctuates a bit between about 1k and 1.2k. When idle drops down into the 700 range, I get a "1", a long pause, then the CEL flutters very quickly, a pause, then the following series:
172
411
538
536
521
[long pause]
[repeat above numbers]
[end test]

My pop suggested an O2 sensor, but code 172 suggests O2 sensing lean to the right side. Is there 2 sensors on this car? Should I swap them to see if the side code changes? I guess the rest of the codes are due to me not interacting with the ER test. Any help appreciated!

Edit: This was resolved. Single upstream O2 sensor replaced ($43). Did a full tune-up. CEL has stayed off now for about 4 days.