DTC Troubleshooting

DTC Troubleshooting: P0420, P0420

DTC P0420:

Catalyst System Efficiency Below Threshold

NOTE:

- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information Initial Inspection and Diagnostic Overview.
- If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot them first, then recheck for DTC P0420.
P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P0300: Random misfire
P0301-P0304: No. 1, No. 2, No. 3, or No. 4 cylinder misfire detected
- Poor quality fuel may cause this DTC.

1. Turn the ignition switch to ON (II).

2. Clear the DTC with the HDS.

3. Start the engine. Hold the engine speed at 3,000 rpm without load (A/T in P or N, M/T in neutral) until the radiator fan comes on, then let it idle.

4. Test-drive under these conditions:

- Engine coolant temperature (ECT SENSOR 1) above 158 °F (70 °C)
- A/T in D, M/T in 4th or 5th
- Vehicle speed between 45-75 mph (72-120 km/h) for 5 minutes or more with cruise control set
- Vehicle speed between 55-75 mph (88-120 km/h) for 10 seconds, then decelerate (with throttle fully closed). Repeat this 3 times or more
- Maintain the vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set

5. Monitor the OBD STATUS for DTC P0420 in the DTCs MENU with the HDS.

Does the screen indicate FAILED?

YES -

Go to step 6.

NO -

If the screen indicates PASSED, intermittent failure, the system is OK at this time. If the screen indicates EXECUTING, keep driving until a result comes on. If the screen indicates OUT OF CONDITION or NOT COMPLETED, go to step 4 and recheck.

6. Turn the ignition switch to LOCK (0).

7. Replace the WU-TWC Warm Up TWC Removal/Installation.

8. Turn the ignition switch to ON (II).

9. Reset the ECM/PCM with the HDS.

10. Do the ECM/PCM idle learn procedure ECM/PCM Idle Learn Procedure.

11. Start the engine. Hold the engine speed at 3,000 rpm without load (A/T in P or N, M/T in neutral) until the radiator fan comes on, then let it idle.

12. Test-drive for about 10 minutes, varying the vehicle speed.

13. Check the DATA MONITOR CONDITION in the DATA LIST with the HDS.

Is the temperature OK?

YES -

Go to step 14.

NO -

Go to step 11 and recheck.

14. Test-drive under these conditions until a result comes on:

- Engine coolant temperature (ECT SENSOR 1) above 158 °F (70 °C)
- Vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set

15. Monitor the OBD STATUS for DTC P0420 in the DTCs MENU with the HDS.

Does the screen indicate OUT OF CONDITION or NOT COMPLETED?

YES -

Go to step 12 and recheck.

NO -

Go to step 16.

16. Check for Pending or Confirmed DTCs with the HDS.

Is DTC P0420 indicated?

YES -

Check the fuel quality, then go to step 1.

NO -

Go to step 17.

17. Monitor the OBD STATUS for DTC P0420 in the DTCs MENU with the HDS.

Does the screen indicate PASSED?

YES -

Troubleshooting is complete. If any other Pending or Confirmed DTCs were indicated in step 16, go to the indicated DTC's troubleshooting.�

NO -

If the screen indicates FAILED, check the fuel quality, then go to step 1. If the screen indicates EXECUTING, keep driving until a result comes on.