Tachometer

• Decreased engine power--indicated by low revolutions per minute (RPMs) on the tachometer--is a symptom of a bad catalytic converter.
  
  

Acceleration

• Another sign of decreased engine power is hesitating or bucking when you try to accelerate.
  
  

Driving Uphill

• The car may seem to lose some power when going uphill.
  
  

Temperature

• If the engine runs hotter than usual but the cooling system is up to par, that may indicate catalytic converter problems.
  
  

Exhaust

• Black smoke coming from the exhaust pipe may be a symptom of a bad catalytic converter.
  
  

Smell

• A failing catalytic converter can cause a rich fuel mixture that makes the exhaust smell like rotten eggs from the hydrogen sulfide.