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Catalyst Efficiency Monitor Federal Test Procedure

The federal test procedure 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 switches are counted under specified conditions for the purpose of calculating rear to front HO2S switch ratio. After the switch ratio is calculated, it is compared against an emission threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. The oxygen storage capacity of a high efficiency catalyst will have a low switch ratio and high HC efficiencies. As catalyst efficiency deteriorates, its ability to store oxygen declines and it will begin to have a higher switch ratio and low HC efficiencies. In general, as catalyst efficiency decreases, the switch ratio increases. Inputs from the Engine Coolant Temp (ECT) sensor, Intake Air Temp (IAT) sensor, and Throttle Position (TP) sensors are required to enable the federal test procedure catalyst monitor. To aid in monitor descriptions. Refer to illustration. See Figure.

  1. In the federal test procedure catalyst efficiency monitor test, only switches during steady state cruise conditions of a drive cycle are counted. Switches at idle or other drive modes are not counted. The counting of front and rear HO2S switches continues until a drive cycle is completed. At that time, the ratio of total rear switches to total front HO2S switches is calculated. If the switch ratio is greater than the emission threshold, the catalyst has failed and a Diagnostic Trouble Code (DTC) is stored. The DTC associated with this test is DTC P0420.
  2. Catalyst Efficiency DTC is stored in memory, and Malfunction Indicator Light (MIL) is turned on after catalyst efficiency monitor detects a malfunction up to 6 consecutive drive cycles.
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