Catalyst Monitoring Diagnostic

Concerns
Concerns associated with the monitoring of catalyst efficiency include the unpredictable effects that exhaust leaks, the use of aftermarket parts, alternate fuels and other factors beyond the manufacturers control may have on the oxygen storage capacity of the catalyst.

Exhaust leaks may adversely affect the diagnostic procedure. Even relatively small leaks can introduce ambient oxygen into the exhaust stream. Depending on their size and location, they may:

^ Prevent a bad catalyst from failing the diagnostic check.
^ Cause a good catalyst to fail.
^ Prevent the diagnostic procedure from even running.

Contamination of the catalyst has always been a concern. Some of the poisons which may be encountered are phosphorous, lead, silica and sulfur. These may prevent the catalyst from working properly and can affect the relationship between oxygen storage capacity and emissions performance. These and other contaminants are typically introduced through the fuel or from the improper use of sealants such as RTV or other shop chemicals.

Vehicles in the Canadian market may be exposed to fuel with an additive known as MMT. MMT affects the oxygen storage capacity of the catalyst monitoring diagnostic.