So my 1999 Subaru Outback, despite having changed the o2 sensors as well as one other sensor the error code reader said was the issue the check engine light is not on nor does it come on or flash. But when I try to smog it the check engine light comes up in the system with no error codes.....any advice?

Check engine light should come on when you turn the key on, but before starting the engine.

 

Once the engine is running, the light should go out.

I don't understand, does the light work or not?

 

Like Gloyale said it should come on when the key is turned to ON or the engine is started. Once the engine is running it should go out after a few seconds. If the light does not turn on for the bulb test it is probably burned out.

 

If there is a stored code post the actual code please.