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Am I correct in thinking these get saved to non-volatile memory and are constantly learned? If so does dis-connecting the battery erase them? Reason for asking is say you have crook O2 sensors and the ECU alters the trims and saves them then when new sensors are fitted it has to go along and take ages to alter them

They are stored in the processors RAM and are lost where power is removed, like when the battery is disconnected or the ECU is disconnected or fuse removed.
In the case of a new O2 it will relearn them, you can reset the ECU by the above methods for fresh re-trimming.