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.
DTC P0420: Catalyst System Efficiency Below Threshold
- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- 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.
- Turn the ignition switch to ON (II).
- Clear the DTC with the HDS.
- Start the engine. Hold the engine speed at 3, 000 rpm without load (in P or N) until the radiator fan comes on, then let it idle.
- Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158°F(70°C)
- 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 the throttle fully closed). Repeat this three or more times
- Maintain the vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set
- 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.
- Turn the ignition switch to LOCK (0).
- Replace the WU-TWC (see WARM UP TWC REMOVAL/INSTALLATION ).
- Turn the ignition switch to ON (II).
- Reset the PCM with the HDS.
- Do the PCM idle learn procedure (see PCM IDLE LEARN PROCEDURE ).
- Start the engine. Hold the engine speed at 3, 000 rpm without load (in P or N) until the radiator fan comes on, then let it idle.
- Test-drive for about 10 minutes, varying the vehicle speed.
- Check the CATA MONITOR CONDITION in the DATA LIST with the HDS.
Is the condition OK?
YES -Go to step 14.
NO -Go to step 11 and recheck.
- Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158°F (70°C)
- Vehicle speed between 55-75 mph (88-120 km/h) for 10 seconds, then decelerate (with the throttle fully closed). Repeat this three or more times.
- Maintain the vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set
- 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 16.
- 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 15, 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. If the screen indicates OUT OF CONDITION or NOT COMPLETED, Go to step 11.
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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.