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EALm4 , no harm in disconnecting the -ve battery post and checking. But if the ECU tree is still all green and no faults registered in ISTA, and you still have the CEL, it's very peculiar... I wonder if there's a specific ISTA procedure to clear them. I'll look tonight.
Update: I found one document on
Malfunction Indicator Lamp (aka CEL) and in there it kinda mentions that it is possible for it to be lit with fault status, but nothing on how to clear.
One thing to try, is the ISTA "OBD Scan Tool", maybe it may display something ?