Engine light is on constantly?
 

Is this a warning for a oxygen/or other sensor?

Could be vast number of reasons for it.
Get your fault codes checked. That will give you a few leads to start from.

If you drop by your local Partsource they will happily loan you the code reader for a few minutes. At least mine here in Toronto does that as a free service. Just plug it in in the slot under the steering wheel and it will spit out a code. Then it's just a matter of a quick google search and you'll know what you are up against. Keep us posted.

Make sure you do follow up. It could be something small or it could be a warning for something bigger. Mine came on recently and I ignored it for a while. Turned out the rad fan was not working properly and I almost overheated.