Toyota Corolla 1970-1987 Repair Guide

Catalytic Converter


See Figures 1 and 2

The catalytic converter is a muffler-like container built into the exhaust system to aid in the reduction of exhaust emissions. The catalytic 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. The oxidizing catalytic converter is used on most late model vehicles. It 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 thermosensor, 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 or 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. Thus, the three-way catalytic converter.

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 cars equipped with an oxygen sensing system.

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Fig. Fig. 1: Oxidizing catalytic converter system

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


  1. Use only unleaded fuel.
  3. Avoid prolonged idling; the engine should run no longer than 20 minutes at curb idle, nor longer than 10 minutes at fast idle.
  5. Reduce the fast idle speed, by quickly depressing and releasing the accelerator pedal, as soon as the coolant temperature reaches 120°F.
  7. Do not disconnect any spark plug leads while the engine is running.
  9. Make engine compression checks as quickly as possible.
  11. Do not dispose of the catalyst in a place where anything coated with grease, gas or oil is present; spontaneous combustion could result.


Testing the catalytic converter operation in the field is a difficult problem. The most reliable test is a 12 hour and 40 min. soak test (CVS) which must be done in a laboratory.

In most cases 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 vehicle is no longer in compliance with Federal or state stands, 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.

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 vehicle to a diagnostic center that has a tester.

If the catalytic converter becomes blocked the engine will not run. The converter has 5 year or 50,000 mile warranty; contact your local Toyota dealer for more information


The warning light comes on while the engine is being cranked, to test its operation, just like any of the other warning light

  1. If the warning light comes on and stays on, check the components of the air injection system as previously outlined. If these are not defective, check the ignition system for faulty leads, plugs, points, or control box.
  3. If no problems can be found in Step 1, check the wiring for the light for shorts or opened circuits.
  5. If nothing else can be found wrong in Steps 1 and 2, check the operation of the emission control system vacuum switching valve or computer, either by substitution of new unit, or by taking it to a service facility which has Toyota's special emission control system checker.