Catalyst Efficiency Monitor
The 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 emission. It 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 intergrated under specified conditions for the purpose of calculating rear to front HO2S voltage ratio. See Fig 1. After the voltage ratio is calculated, it is compared against an emission threshold value for a "healthy" catalyst. If rear to front intergrated 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 intergrated voltage ratio and low HC efficiencies. In general, as catalyst efficiency decreases, the intergrated rear to front voltage ratio decreases. Inputs from the ECT, IAT and TP sensors are required to enable the catalyst monitor.
- When the rear to front intergrated voltage ratio is too low when compared to the emission threshold value for a deteriorated catalyst; the catalyst has test fails. The DTC associated with this test is DTC P0421.
- Catalyst Efficiency DTC is stored in memory, and Malfunction Indicator Light (MIL) is illuminated after catalyst efficiency monitor detects a malfunction up to 3 consecutive drive cycles.