The O2 sensor should toggle between .1 and .9 volts about every other second when the engine is in closed-loop operation.
Some digital meters in particular are usually not the best for measuring varying voltages; the rate they sample at, how much they average (integrate) the reading voltages, and how often they update the display all have an effect on what you read. The ideal reading for an O2 sensor is .45V, but due to the imprecise processes of the combustion engine this reading is impossible to maintain, and the ECU is constantly trying to compensate.
More to the point, however, is that your sensor 'seems' to be working from what you've said so far. If it goes toggles high and low that is a good sign. (If it was stuck high or low you would get an ECU code about fuel trim, etc.) They don't like shock, though, so yes it is entirely possible you ruined the sensor in some way by banging on the exhaust, like maybe the heater is broken.
Anyway, I would look for something that might have been part of the A/C work, like maybe a spark plug wire not seated or broken, etc. first.
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