The catalytic converter is mounted in the engine exhaust stream ahead of the muffler. Its function is to combine carbon monoxide (co) and hydrocarbons (hc) with oxygen and break down nitrogen oxide (nox) compounds. These gasses are converted to mostly co 2 and water. It heats to operating temperature within about 1-2 minutes, depending on ambient and driving conditions and will operate at temperatures up to about 1500°F. Inside the converter housing is a single or dual bed ceramic monolith, coated with various combinations of platinum, palladium and rhodium.
The catalytic converter is not serviceable. If tests and visual inspection show the converter to be damaged, it must be replaced. There are 2 types of failures: melting or fracturing. The most common failure is melting, resulting from unburned gasoline contacting the monolith, such as when a cylinder does not fire. Usually when the monolith melts, high backpressure results. When it cracks, it begins to break up into small practices that get blown out the tail pipe.
Poor fuel mileage and/or a lack of power can often be traced to a melted or plugged catalytic converter. The damage may be the result of engine malfunction or the use of leaded gasoline in the vehicle. Proper diagnosis for a restricted exhaust system is essential before any components are replaced. The following procedure that can be used to determine if the exhaust system is restricted.
Check at Oxygen Sensor
- Carefully remove the oxygen sensor.
- Install an adapter that has the same threads as the sensor and that will hook up to a pressure gauge. Install in place of the sensor.
- Perform Backpressure Diagnosis Test, given below.
- When test is complete, remove the pressure gauge and adapter. Lightly coat the threads of the oxygen sensor with an anti-seize compound. Reinstall the oxygen sensor.
- With engine idling at normal operating temperature, observe the backpressure reading on the gauge. The reading should not exceed 1.25 psi (8.6kPa).
- Increase engine speed to 2000 rpm and observe gauge. The reading should not exceed 3 psi (20.7kPa).
- If the backpressure at either speed exceeds specification, a restricted exhaust is indicated.
- Inspect the entire exhaust system for a collapsed pipe, heat distress or possible internal muffler failure.
- If there are no obvious reasons for the excessive backpressure, the catalytic converter is suspected and should be removed for inspection or replacement.
- Raise and safely support the vehicle.
- Inspect the catalytic converter protector for any damage.
If any part of the protector is dented to the extent that is contacts the converter, replace it.
- Check the heat insulator for adequate clearance between the converter and the heat insulator. Repair or replace any damaged components.