Q: Surely it's a malfunction if some days the light is on and other days the light is NOT on. Right????? It's hard to take it seriously if the darn thing can't make up its mind.
A: Late model Volvo's are like that. To clear my check engine light I had ti replace a $30 Evaporative purge valve and it's rubber hose. I own a '01 S80 T6. When the light comes on use a scanner to pull the codes. Autozone sells generic scanners fro about 50 bucks right now and they are an excellent investment.