I know what you mean it sucks wasting money on something not needed. But you said you had codes from the O2 sensor.
Just out of curiosity what did the metal cage on the Bosch O2 sensor look like that you replaced on the galant with no issues you are refering to?
Reason I ask is I just replaced mine a few months back and the stock O2 sensors metal protector look like swiss cheese sort of speaking. Where the new Bosch one only had 3 or 4 slots around it. And my gas millage is very low with the new Bosch one. And I have noticed when I leave my house on a cold day the car seems to stay in a so called "choke" position for a longer period of time then when I had my stock O2 on when it was working.
Check this thread out if you dont mind. I am going to try adding a little more to it.
https://www.thegalantcenter.org/showthread.php?t=46297
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