Possible vaccuum leak

I recently bought a 95 tracer trio the muffler pipe was broke between the flex pipe and the resinator. I had an oil change and ever since the oil change when the car idles when it is warm the check engine light comes on. When I start driving it will go off. Then when I stop at a light it comes back on. I was told it was the exshust and had fixed but symptom is still their. I was told that it could be a vaccuum leak. Can a code be found if it happens when it idels to tell me whats wrong. I was told no by a friend. Can you help and tell me what the problem may be wrong oil maybe they put in 10w30 instead of 5w30. It also idels between 550 & 700 rpms. And seems to idle a little rough but that did that when I bought the car. I had the car about 3 or 4 days before the oil change. I also put in some fuel injection cleaner made by gumout for high milage cars and filled it half way with regular unleaded could it also be bad gas. I did the exhust repair two days after the oil change. :X