Black smoke from the exhaust of a 1997 Toyota Camry.

There has been black fumes coming from my cars exhaust pipe whenever the engine is running and when I press on the accelerator to rev the engine. Also, the engine's consuming more fuel than usual. The sound of the engine is also deeper and louder than it usually is. The engine check light was on and I took it to the mechanic. 3 error codes were generated but the mechanic insisted that just changing the muffler in the exhaust would end the problems. He however did not tell me what the problems the 3 error codes indicated. Please what can I do?