DTC P0420: Catalytic System Efficiency Below Threshold NOTE: If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot those DTCs first, then recheck for DTC P0420. P0137, P0138: Secondary Heated Oxygen Sensor (secondary HO2S) (Sensor 2) P0141: Secondary HO2S (Sensor 2) heater Reset the ECM/PCM (see page 11-4), then continue to step 2 through 5 to reset the readiness code. Start the engine. Hold the engine at 3,000 rpm (min-1) with no load (in Park or neutral) until the radiator fan comes on. Drive for about 10 minutes without stopping on a highway or freeway. Your speed can vary. With the A/T in D position, M/T in 4th gear, drive at a steady speed between 80-100 km/h (50-62 mph) for 30 seconds. Repeat step 4 three times. Between each repetition, close the throttle completely for 1-2 seconds. If the engine is stopped during this part of the procedure, go to step 3 and do the procedure again. Check for a Temporary DTC with the scan tool. Does the scan tool indicate Temporary DTC P0420? YES - Check the TWC. If necessary, replace the TWC. NO - Check for readiness code completion. If the readiness is complete, it was a intermittent failure, system is OK at this time. If the readiness is incomplete, repeat steps 2 through 5.

Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on, then let it idle. Connect a tachometer. Check the idle speed (see page 11-125). Warm up and calibrate the CO meter according to the meter manufacturer's instructions. Check idle CO with the headlights, heater blower, rear window defogger, cooling fan, and air conditioner off. The CO meter should indicate 0.1 % maximum.