Signs of a faulty catalytic converter include reduced fuel economy, excess engine heat, and impaired engine or exhaust performance. A visible fuel vapor over the carburetor intake, performance changes after the removal of oxygen sensors and increased carbon emissions from the converter are also warning signs.
The purpose of the catalytic converter is to change harmful gases into safe compounds that the exhaust system can send into the atmosphere. When catalytic converters fail, the cause is generally impact from debris or overheating.
Cars with catalytic converter failure generally show it first through an impairment in engine performance. The car's fuel economy dwindles significantly, making more frequent trips to the gas station necessary. Catalytic converters that trap exhaust ahead of the catalyst lead to back pressure, making the engine stall while being driven and causing the car to jerk when the accelerator is depressed.
Experienced mechanics can test the catalytic converter by removing the oxygen sensors from the exhaust and operating the car. A significant difference in performance indicates a clog in the catalytic converter. Elevated carbon emissions through the exhaust system show in the form of dark smoke and mean that antifreeze, motor oil or another foreign chemical has made its way into the catalytic converter.