A malfunctioning catalytic converter can create major problems for your vehicle. It causes your engine to run less efficiently, which will reduce power. The catalytic converter also changes carbon monoxide, hydrocarbons and nitrogen oxides emitted by your vehicle into nitrogen, oxygen, carbon dioxide and water. When it doesn't work, the compounds exhausted from your vehicle are much more harmful to the environment. Replacing a catalytic converter is usually expensive, so it's important to verify the symptoms do not indicate a different problem.
Watch the tachometer for readings lower than normally produced by your vehicle. The decreased engine power from a bad catalytic converter will cause the tachometer to show a lower rpm reading.
Track the gas mileage obtained by your vehicle. Automotive website AA1Car says catalytic converter problems often reduce a vehicle's fuel economy. If your car requires more gas to travel familiar distances, the catalytic converter could be the cause.
Observe the engine temperature of your vehicle. The reduced engine efficiency of a vehicle with catalytic converter problems causes energy that would have been used to power the vehicle to instead turn into heat. The temperature of the engine will go up or down as the vehicle moves. But when the catalytic converter has problems, the temperature will stay higher than it normally would while moving at a constant speed.
Accelerate the vehicle. The easiest way to spot catalytic converter problems is by paying close attention when the car accelerates. Watch for bucking or stuttering motions as the car or truck moves forward. The vehicle may hesitate to move for a moment when you press the accelerator. This is usually followed by a strong jolt as the vehicle jerks forward. A bad catalytic converter may cause the vehicle's engine to stall. It will usually start without trouble and otherwise seem fine but stall immediately when the accelerator is pressed.
Look at the exhaust from your vehicle. This would normally be filtered by the catalytic converter. More smoke being emitted than usual points to problems with the catalytic converter. Some vehicles with catalytic converter problems emit black smoke.
Smell the air behind the car. Malfunctioning catalytic converters often emit hydrogen sulphide. This compound smells like rotting eggs and can be strong.