Hi I have a 2000 S40 1.9T and I was wondering why the check engine light is on it has its moments comes on for a few weeks then shuts off for a couple weeks but its been on for a while any hints or advises I would appreciate it!

Thanks!

You need to have the codes pulled on it.When is the last time you had it to the dealer for work?Has it had the latest software and the o2 sensor pigtail installed?

As tech has said, the check engine light is on to tell you that theres something wrong. "having the codes pulled on it" means have the onboard diagnosis computer tell you what is wrong. (this is done through codes) stop by the dealership and have them give you the codes, from there we can try to narrow down your problem