Toyota Celica/Supra 1971-1985 Repair Guide

Catalytic Converter



See Figures 1, 2 and 3

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Fig. Fig. 1: Oxidizing catalytic converter system

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Fig. Fig. 2: Three-way catalytic converter system

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Fig. Fig. 3: Three-way catalytic converter system with a thermo sensor

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 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 like water and carbon dioxide.

There are essentially two types of catalytic converters: an oxidizing type and a three-way type. Both are used on the late model Celicas. The oxidizing catalyst 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 .

An air injection system is used to supply air to the exhaust system to aid in the reaction. A thermo sensor, inserted into the converter, shuts off the air supply if the temperature of the catalyst becomes excessive.

The same sensor circuit will also cause an instrument panel warning light labeled "EXH TEMP" to come on when the catalyst temperature gets too high.

It is normal for the light to come on temporarily if the car is being driven downhill for long periods of time (such as descending a mountain).

The light will come on and stay on if the air injection system is malfunctioning of if the engine is misfiring.

The oxidizing catalytic converter, while effectively reducing HC and CO emissions, does little, if anything, in the way of reducing NOx emissions.

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 later cars equipped with an oxygen sensing system.


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