I have a 2005 honda civic, the check engin ligth came on serveral months ago then it went away. I would normally get 520km on a full tank , but just prior to the light coming on I got 800km on a full tank. I'm now getting 1000km on a full tank.. the engin check ligth is still on (over 9 weeks now). I am doing any damage to my engin.

You really need to hook a scan tool or code reader up to the Data Link Connector and find out what's going on. There's obviously a problem somewhere.

You need to hook up a code reader to the OBDII port and scan it to see what the problem is. The OBDII is normally located under the dash on the driver side. If you don't have a code reader than you can take it to a local Autozone or Oreilly Auto Part store. Hope this helps.

The reason the light is on is because a component part is not functioning properly. The computer reacts by causing other components to function irregularly to compensate for the faulty component. So the longer you wait, the more strain is put on other parts unnecessarily. You need to have it diagnosed and repaired, or you'll just end up costing yourself more in the long run.