Emission System Service Required
 

I have a 1999 S80 that I bought used and has given me constant trouble. The check engine light comes and goes but has decided to stay on for a while now. Also, I will get a random message "emission system service required" that will come and go too. Only today, I was driving and noticed that while sitting idle, the car had a strange vibration, and the ESSR light came back on. While driving the car is smooth.

Help! I'm a student living on very little money and I really can't afford something else to drain my account.