Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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.
- 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.
- 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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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.