It's not easy to diagnose without a mechanic, but numerous symptoms can point to a bad catalytic converter. The catalytic converter's purpose is to change hydrocarbons, carbon monoxide and nitrogen oxides in vehicle emissions into the harmless compounds of carbon dioxide, nitrogen, oxygen and water. Catalytic converters don't typically fail, but they can become clogged.
Decreased engine power--indicated by low revolutions per minute (RPMs) on the tachometer--is a symptom of a bad catalytic converter.
Another sign of decreased engine power is hesitating or bucking when you try to accelerate.
The car may seem to lose some power when going uphill.
If the engine runs hotter than usual but the cooling system is up to par, that may indicate catalytic converter problems.
Black smoke coming from the exhaust pipe may be a symptom of a bad catalytic converter.
A failing catalytic converter can cause a rich fuel mixture that makes the exhaust smell like rotten eggs from the hydrogen sulphide.