The Nissan Path

So, i've noticed that my exhaust pipe tip has been getting darker and soot has been covering it. I don't know if this has anything to do with the fact that while I'm breaking, my oil light and check engine come on and then disappear once I let go or begin letting go of the brake pedal. Any ideas? Would really like to keep this Pathy. I love this vehicle!! It's a 98' 4x4 Auto SE

Checked for any codes?

The guy at Autozone ran it and didn't "remember," the code. He just said it was a faulty knock sensor?? I'm going to a different autozone with employees who pay more attention.