Help with "ER xx" codes on MY06 3.0L Subaru Outback

Hi .. complete newbie to forums/groups ... have had a look already but
can only find limited references to "ER" codes. I received an "Er AH"
code on the odometer of my MY06 3L Outback a few days ago. No other
warning lights although analogue engine temperature gauge was on
maximum. This happened once before in the 8 months (and 21000km) that I
have had the vehicle but I can't remember if it was "Er AH" or "Er AT".
AT the time I took it to a dealer who couldn't find anything.

At the time I was driving on freeway at 100km and had done about 25km.
No hard driving (although I do hammer it in sports mode from time to
time).

Pulled to side of freeway and let engine idle with bonnet up as per
drivers manual. Checked engine fans .. all working. After fifteen
minutes no change so turned car off and on again. All ok and
temperature at about half.

Rang dealer who had no idea what an "Er AH" was.

Can anyone help?

Thanks.