The engine is running rich or the catalytic converter is partially clogged, the converter can be checked easily enough, if tha checks out ok then one or more of the engine controls are not working correctly, most specifically the sensors that have the most impact on the air/fuel ratio; throttle position (TPS), manifold absolute pressure (MAP) or mass airflow sensor (MAF) and the oxygen sensor (O2).Every sensor can be checked for correct operation with common tools, you'll need a high impedance multimeter, vacuum gauge and some knowledge to do your own repairs..