GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

Catalytic Converter


See Figure 1

The catalytic converter is a muffler-like container built into the exhaust system to aid in the reduction of exhaust emissions. The catalyst element consists of individual pellets or a honeycomb monolithic substrate coated with a noble metal such as platinum, palladium, rhodium or a combination. When the exhaust gases come into contact with the catalyst, a chemical reaction occurs which will reduce the pollutants into harmless substances including water and carbon dioxide.

There are essentially two types of catalytic converters: an oxidizing type and a three-way type. The oxidizing type requires the addition of oxygen to spur the catalyst into reducing the engine's HC and CO emissions into H 2 O and CO 2 . The oxidizing catalytic converter, while effectively reducing HC and CO emissions, does little, if anything in the way of reducing NOx emissions. Thus, the three-way catalytic converter.

Click image to see an enlarged view

Fig. Fig. 1: The catalytic converter is a muffler-like device built into the exhaust system to aid in the reduction of exhaust emissions - 1985 Celebrity wagon shown

The three-way converter, unlike the oxidizing type, is capable of reducing HC, CO and NOx emissions, all at the same time. In theory, it seems impossible to reduce all three pollutants in one system since the reduction of HC and CO requires the addition of oxygen, while the reduction of NOx calls for the removal of oxygen. In actuality, the three-way system really can reduce all three pollutants, but only if the amount of oxygen in the exhaust system is precisely controlled. Due to this precise oxygen control requirement, the three-way converter system is used only in conjunction with an oxygen sensor system.

There are no service procedures required for the catalytic converter, although the converter body should be inspected occasionally for damage.


Use only unleaded fuel.
Avoid prolonged idling; the engine should run no longer than 20 minutes at curb idle and no longer than 10 minutes at fast idle.
Do not disconnect any of the spark plug leads while the engine is running.
Make engine compression checks as quickly as possible.


At the present time there is no known way to reliably test catalytic converter operation in the field. The only reliable test is a 12 hour and 40 minute soak test which must be done in a laboratory.

An infrared HC/CO tester is not sensitive enough to measure the higher tailpipe emissions from a failing converter. Thus, a bad converter may allow enough emissions to escape so that the car is no longer in compliance with Federal or state standards, but will still not cause the needle on a tester to move off zero.

The chemical reactions which occur inside a catalytic converter generate a great deal of heat. Most converter problems can be traced to fuel or ignition system problems which cause unusually high emissions. As a result of the increased intensity of the chemical reactions, the converter literally burns itself up.

A completely failed converter might cause a tester to show a slight reading. As a result, it is occasionally possible to detect one of these.

As long as you avoid severe overheating and the use of leaded fuels it is reasonably safe to assume that the converter is working properly. If you are in doubt, take the car to a diagnostic center that has a tester.