Originally Posted by signes
A leak would generate a CEL, but you should get it fixed regardless. Where is it? You have both the primary and secondary stock cats still on? You could also have a faulty O2 sensor among about 1000 other reasons.
A leak in the exhaust system may not trigger any CEL - depends where it is.
You probably have an o2 sensor that has a problem, if you're confident there isn't a leak somewhere.