TECHNICAL - WHAT CAUSES A CONVERTER FAILURE?

You remove a defective converter from a car and it has melted down. What could have caused the converter to melt down? A step by step approach is needed to find the answer, so let's start at the beginning.

FUEL
Unleaded fuel must be used in vehicles with a catalytic converter. Using leaded fuel will cause the converter to become plugged and inoperative. Also some gas additives can damage a converter. Always check the instruction label of any additive added to your fuel system to be sure that it is converter safe!

OXYGEN SENSORS
This sensor sends information to the ECU (Electronic Control Unit) to maintain proper air/fuel mixture. It is located either in the exhaust manifold pipe or catalytic converter. As an oxygen sensor operates, it checks to see if oxygen is present in the system. If no oxygen is detected in the system, this would indicate a rich mixture, the oxygen sensor will read about 900mV (millivolts). When excessive amounts of oxygen are present, the sensor voltage will read about 100mV Rich mixtures mean high readings, lean mixtures mean low readings. Please note the exhaust temperature must be at least 600 degrees fahrenheit for the sensor to operate, and the measurements should only be taken with a multimeter that has a 10 meg ohm impedance rating. Failure to follow this rule may cause you to replace the vehicle's ECU. A properly functioning sensor will oscillate between 900mV and 100mV every few seconds. You can measure this with your multimeter. If the reading does not oscillate or stays fixed, the sensor is defective. Also note that silicone from either antifreeze or RTV (silicone) sealant, can clog an oxygen sensor and the mixture rich and in time will ruin the combustion chamber the sensor should be replaced. When using a silicone sealer, make sure it is oxygen sensor safe.

CARBURETTOR SYSTEMS
A carburettor is a device that mixes petrol with air and delivers it Into the intake manifold for the engine to burn. If a carburettor is worn or defective it may be the cause of a converter failure. Problems such as improper float adjustments, air/fuel mixture adjustments, out of spec or worn metering rods can cause damage to the catalyst. Also check the choke system to make sure it is operating properly and if the carburettor is an electronic feedback model make sure all adjustments are made according to manufacturer specs and that all feedback sensors or electronic components, such as mixture control solenoids, throttle position sensors and throttle holding solenoids are also operating and are within specs.

FUEL INJECTION SYSTEMS:
Problems in fuel injection systems can also cause problems with the converter. Fuel injection systems differ greatly. Basically a fuel injection system uses small electrically controlled solenoids that deliver or inject fuel into the throttle body or into the intake part of the engine. These little solenoids are controlled by a computer which obtains information from various sensors located through out the engine. These sensors help the computer to determine how much fuel to inject. If an injector is leaking internally or dribbling fuel into the engine it may damage the converter. If the oxygen sensor is defective or the map sensor is bad this can also lead to converter damage. If the injection system uses a cold start injector (which is a small injector that is used to richen the mixture by injecting fuel into the intake air stream when the engine is cold) and if this injector is leaking or dripping, or if it's temperature sensor is not operating properly this too can damage the converter. Because of the many different injection systems used, consult the proper manual when diagnosing this system.

IGNITION SYSTEMS:
Many parts are used in the ignition system. This system delivers the voltage or spark to the appropriate cylinder to fire the engine. Any of the following problems can cause the converter to fail or be damaged, fouled spark plugs, bad spark plug wires, carbon tracked distributor cap or rotor cracked distributor cap. Make sure all spark plugs and wires are firing properly. A misfiring plug not only wastes gas but it make it defective. If the car has a coolant leak into makes converter. Be sure to check distributor timing and vacuum advance for proper operation.

MAP SENSORS:
These sensors tell the ECU how much air is entering the engine as well as the load on the engine. It also monitors barometric pressure. When this sensor fails it can cause a rich condition which can damage the converter. Because testing of these sensors vary between make and model, consult the manual before testing. Generally there are two types, the voltage type and the frequency varying type. To check the voltage type you would use a volt meter and vacuum pump. To check the frequency type you would use a tach and a vacuum pump.

CANISTER PURGE VALVE:
This vacuum-operated valve vents fuel vapours from the carburettor bowl to the charcoal canister. If the diaphragm becomes ruptured, the charcoal canister will flood and the mixture becomes rich and can damage the converter. To check the valve, disconnect the hoses to see if any petrol comes out of the hoses or valve. If there is petrol the valve should be replaced.

EGR SYSTEM
This system routes small amounts of exhaust gasses back to the Intake manifold to reduce oxides of nitrogen or NOX. Systems vary by design so you need to consult the proper manual for the vehicle you are working on. Some newer systems also use an EGR valve position sensor. If the exhaust system you are working on has one it must be checked also. While a bad EGR system will not lead to a converter failure, it might be the reason why the vehicle has a NOX problem.