White smoke after aos replacement

[lesd]

Hi.
First post on the forum and looking for some advice.
The girlfriends 2.5 1997 Boxster started billowing white smoke out of the exaust. After following all the good advice on this and other websites, i replaced the aos valve and the smoke stopped. The old aos valve had the cream slime of oil/ water in it. 3 days later, the car has again began to billow smoke and the exaust is a glazed black colour, but no wet oil residue. The oil and coolant levels are fine and the temperature is ok. Before replacing the aos the orange ecu light had come on, but since the replacement has gone out. Any ideas what it could be, such as head gasket or hopefully something less drastic. Really appreciate the help. Cheers.