Went lean...

The O2 sensor doesn’t know if it’s carb’d or EFI. It’s a dumb sensor.

Did the engine behave differently? I always believe the plugs over the O2 sensor. It doesn’t know the difference between a lean miss rich or just rich. You have to be able to differentiate between the two.

If it’s actually lean then you need to look for a vacuum line that got loose or something like that.

If it’s not actually lean then you need to address the sensor and such.