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GM Prizm/Nova 1985-1993 Repair Guide

3-Way and Oxidation Catalyst (TWC-OC) System



The catalytic converter is a muffler-like container built into the exhaust system to aid in the reduction of HC, CO and NOx emissions by changing them into nitrogen, carbon dioxide and water vapor through the action of the catalyst upon the exhaust gas.

The 3-way catalytic convertor is the best type to use since it can change all 3 types of emissions into non-polluting gases. In this type of converter nitrous oxides are chemically reduced by the catalyst and reformed into the molecules of oxygen and nitrogen. The oxygen formed by the reduction reaction is then used to oxidize carbon monoxide and the hydrocarbons, forming carbon dioxide and water vapor.

For the catalytic converter to work most efficiently, the following conditions must be met:

Operating temperature must be over 500°F (260°C).
Air/fuel ratio must be held closely at 14.7:1.


Use only unleaded fuel.
Avoid prolonged idling; the engine should run no longer than 20 minutes at curb idle, nor longer than 10 minutes at fast idle.
Reduce the fast idle speed, by quickly depressing and releasing the accelerator pedal, as soon as the coolant temperature reaches 120°F (49°C).
DO NOT disconnect any spark plug leads while the engine is running.
Always make engine compression checks as quickly as possible. Excess fuel can be pumped through the motor and build up in the converter.
DO NOT dispose of the catalyst in a place where anything coated with grease, gas, or oil is present; spontaneous combustion could result.
Since the inside of the catalyst must reach 500°F (260°C) to work efficiently, the outside of the converter will also become very hot. Always be aware of what may be under the car when you park. Parking a hot exhaust system over dry grass, leaves or other flammable items may lead to a fire.