Catalyst Efficiency Monitor
PURPOSEThe Catalyst Efficiency Monitor is an on-board strategy designed to monitor and determine when a catalytic converter has fallen below the minimum level of effectiveness in its ability to control exhaust emissions.
OPERATION
The monitor relies mainly on the front and rear Heated Oxygen Sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity The front and rear HO2S voltages are integrated under specified conditions for the purpose of calculating a rear to front HO2S voltage ratio. After the integrated voltage ratio is calculated, it is compared against a minimum emission threshold value for a "healthy" catalyst. If the rear to front integrated voltage ratio is less than the emission threshold, the catalyst has failed. As catalyst efficiency deteriorates, its ability to store oxygen declines and it will begin to have a lower rear to front integrated voltage ratio and low HC efficiencies. In general, as catalyst efficiency decreases, the integrated rear to front voltage ratio decreases. Inputs from the Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), Throttle Position (TP) sensors are required to enable the Catalyst Monitor.
DTCs and MIL Operation
Catalyst Efficiency Monitor:
1 When the rear to front HO2S integrated voltage ratio is too low when compared to the emission threshold value for a deteriorated catalyst; the catalyst test fails.
The DTC associated with this test is DTC P0421.
2 The MIL is activated after three sequential failures of the catalyst efficiency test has been detected.