i replaced the o2 sensor and then had the check engine reset when i went for smog the guy pluged something into my obd and said that the modules or something had not reset yet and that i need to go drive it around well after 150 miles still having the same problem...is this normal? anyone know how long it takes for this to go away (the check engine light is off and no codes are being thrown/)

 

Not to familiar with the S40s but I know on my 850, there was a very specific drive trace (drive sequence) and it did not always work and could have taken 5k or better. Volvo had several TSBs about it

 

If you take it to be tested and it flags they will likely grant an extension and you go back later. I'd call the test facility to see how they handle those situations.