Ford Mid-Size Cars 1971-1985 Repair Guide

Catalytic Converter System


See Figure 1

Starting in 1974 most models have catalytic converter(s) located in the exhaust system. The

converter works as a gas reactor, and its catalytic function is to speed up the heat producing chemical reaction between the exhaust gas components in order to reduce the air pollutants in the engine exhaust.

The catalyst material is contained in a sealed, honeycombed chamber. It is the surface of the catalyst material that plays a major role in the heat-producing chemical reaction. There are basically three types of catalytic converters:

  1. The Conventional Oxidation Catalyst (COC); used to oxidize hydrocarbons (HC) and carbon monoxide (CO).
  3. The Three-Way Catalyst (TWC); not only works on HC and CO but also reduces nitrogen oxides (NOx).
  5. The Light Off Catalyst (LOC); arranged in series with the main catalytic converter, is designed to handle the exhaust emissions during engine warm-up when the main converter has not reached the proper temperature for maximum efficiency.

In order to provide the oxygen necessary to obtain the converter's maximum efficiency a secondary air source is provided by the air pump (Thermactor®). The system is protected by several devices that block out the secondary air when the engine is laboring under any abnormal hot or cold operating situation.

The catalytic converter is expected to function without service for at least 50,000 miles (80,400 km). Use of leaded fuel would quickly cause catalyst failure and an expensive repair bill.

Click image to see an enlarged view

Fig. Fig. 1: Cut-away of a catalytic converter


Naturally, lead-free fuel must be used in order to avoid contaminating the converter and rendering it useless. However, there are other precautions which should be taken to prevent a large amounts of unburned Hydrocarbons (HC) from reaching the converter. Should a sufficient amount of HC reach the converter, the unit could overheat, possibly damaging the converter or nearby mechanical components. There is even the possibility that a fire could be started. Therefore, when working on your car, the following conditions should be avoided:

The use of fuel system cleaning agents and additives.
Operating the car with a closed choke or a submerged carburetor float.
Extended periods of engine run-on (dieseling).
Turning OFF the ignition with the car in motion.
Ignition or charging system failure.
Misfiring of one or more spark plugs.
Disconnecting a spark plug wire while testing for a bad wire or plug, or poor compression in one cylinder.
Pushing or tow-starting the car, especially when hot.
Pumping the gas pedal when attempting to start a hot engine.