Dealer Inspection Proof

Hi all, I bought a used Toyota from a dealer last month. And last week the my car failed the state emission/inspection as it had a check engine light. They gave me an error code and said it need to be replaced (Oxygen sensor) with a failed report that requires the car to be fixed and inspected by a authorized service center. My dealer claims to have got my car repaired and it has now 'passed' the inspection (the car now has a passed sticker). I asked him a warranty for the replaced part, but my dealer doesn't give any. He just tells me to bring the car back if I have a problem. How can I be sure they just didn't reset the check engine light and got the emission tested. And even if I get a check engine again in couple of weeks, will my dealer refuse to fix it because the initial warranty on the car was for only one month or 5000 miles. Kindly enlighten me the right approach that I need to follow to address my situation.
Regards, AJ

As long as the problem accures in the warranty period, and its says any problems will be fixed, then they have to fix it. Make sure they replaced the sensor, because after the warranty expires the car will be a AS/IS car and then you will need to pay out of pocket to replace/fix, thats how it is