Advanced Diagnostics

Advanced Diagnostics

DTC P0420: Catalyst System Efficiency Below Threshold (Engine Type: K20Z3)






General Description
The three way catalytic converter (TWC) converts hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) in the exhaust gas to water vapor, carbon dioxide (CO2), and dinitrogen (N2). The engine control module (ECM) fluctuates the air/fuel ratio (A/F) temporarily to detect the purification performance of the TWC and measures the degree of fluctuation in the secondary HO2S output. If the accumulated value for a set time is too great, deterioration of the performance of the catalyst is detected.

Monitor Execution, Sequence, Duration, DTC Type, OBD Status






Enable Conditions






Malfunction Threshold
The secondary HO2S fluctuation integration value is 3,300 mV or more for at least 10 seconds.

Driving Pattern






1. Start the engine. Hold the engine speed at 3,000 rpm without load (in neutral) until the radiator fan comes on.
2. Drive the vehicle at a speed between 45 - 75 mph (73 - 120 km/h) for at least 5 minutes, to warm up the TWC.
3. Then, drive at steady speed between 55 - 75 mph (88 - 120 km/h) for at least 10 seconds.
4. Decelerate with the throttle valve fully closed.
5. Repeat Driving Patterns 3 through 4, four times.
6. Set a vehicle speed of 55 mph (88 km/h) on the cruise control, and drive for at least 70 seconds.

- Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

Diagnosis Details

Conditions for illuminating the MIL
When a malfunction is detected during the first drive cycle, a Temporary DTC is stored in the ECM memory. If the malfunction recurs during the next (second) drive cycle, the MIL comes on and the DTC and the freeze frame data are stored.

Conditions for clearing the MIL
The MIL will be cleared if the malfunction does not recur during three consecutive trips in which the diagnostic runs. The MIL, the DTC, the Temporary DTC, and the freeze frame data can be cleared by using the scan tool Clear command or by disconnecting the battery.