Reasons For Failure
When a catalytic converter fails it is usually due to a fault with the vehicle that must be rectified otherwise the replacement unit is also likely to fail.
There are many reasons why a catalytic converter can fail but below we have listed the most common causes.
A faulty lambda sensor will cause the engine to over fuel thus damaging the catalytic converter.
Faulty spark plugs, HT leads or ignition coils will cause incomplete combustion and the unburned fuel entering the exhaust will damage the catalytic converter.
The P0420 fault code (catalyst below efficiency) can mean that your catalytic converter is under-performing but it can also mean that your lambda sensor is detecting an incorrect air/fuel mixture which could be caused by incomplete combustion or by an air leak upstream of the sensor.
A worn engine will pass oil into the combustion chamber which cannot be fully burned thus entering the exhaust system and coating the monolith of the catalytic converter causing it to under-perform.
If the catalytic converter body is damaged it is likely that the internal monolith has been fractured which will cause it to under-perform.
MOT Emissions Test Failure
If you are replacing your catalytic converter because your vehicle has failed the MOT emissions test please check the emissions failure report against the information listed below.
If a car fails the emissions test due to an under-performing catalytic converter the only reading that will be too high will be the CO percentage.
If the HC level is too high this relates to a poorly performing engine. HC is unburned, or partially burned hydrocarbons. In gas analysis, these hydrocarbons can come from two sources. Firstly, the engine may be worn and passing oil into the combustion chamber, which cannot be fully burned. Secondly, the engine may have developed a misfire, or suffer from only partial combustion of the fuel. This unburned fuel registers as HC on the gas analyser (hydrocarbons, is an indication of unburned fuel exiting the exhaust, the more incomplete the combustion, the higher the HC reading). Please be aware that even though the MOT emissions test allows up to 200 ppm for the HC reading any reading higher than 50 means that the vehicle has a problem that needs rectifying.
If a car fails the Lambda test this usually means that either the engine is running very rich or has an air leak. The perfect Lambda reading is 1. A reading lower than 1 shows that a car is running rich. A reading higher than 1 shows that the engine is running lean or that there is an air leak in the system. A vehicle with a high HC reading and a low lambda reading will almost certainly have high CO but the cause is unlikely to be a faulty catalytic converter.
Removing The Old Catalytic Converter
Catalytic converters operate at a very high temperature and retain the heat for a long period so before removing the old part ensure that it has fully cooled down.
Unplug the wiring to the lambda sensor if it is located in the converter assembly and unscrew it carefully because they have a tendency to seize and are easily damaged.
When fitting a converter follow the normal exhaust fitting procedure, but heed the following warnings:
Always offer the new catalytic converter up to the car to see if it will fit. If you feel the catalytic converter will need to be altered to enable installation contact our technical department as you may have the wrong item for your car. Any alterations made without the prior authorisation of the manufacturer will invalidate the warranty.
Use new gaskets whenever possible because an air leak into a catalytic converter can at best cause an MOT emission failure and at worst this can cause premature converter failure. A common cause of the P0420 (catalyst below efficiency) fault code is an air leak in the exhaust system.
Never use exhaust paste in front of the catalytic converter. The chemicals used in the manufacture of exhaust pastes can have an adverse effect on the monolith of the catalytic converter causing premature failure.
Never hit a catalytic converter with hammer, mallet or chisel gun to try and force it into position. Doing so may fracture the monolith and invalidate the warranty.
Refit the lambda sensor observing the manufacturers torque settings (do not over tighten) and reconnect the wiring as necessary.
Run the engine and check all joints are fully tightened and are not leaking.