Brilliant, absolutely brilliant result - and a surprise. I didn't think it stored hard errors and flagged them up on the dash, until cleared via the OBD. It would be worth your while scanning it for faults now, very occasionally, just to see if any more codes are flagged up. It costs you nowt and it can often pre-empt a breakdown.
Before removing the oil switch I disconnected battery for long time and even during thred repair battery was disconnected. The system was drained of all power but error never disappeared. That's why did not think about the ECU. Always something to learn.