DTC Troubleshooting

P0420 DIAGNOSIS PROCEDURE

Step	Action	YES	NO
1	Was "Engine and Emission Control System Check" performed?	Go to Step 2.	Go to ENGINE AND EMISSION CONTROL SYSTEM CHECK:J24B .
2	DTC check Check DTC. Is there DTC(s) other than P0420?	Go to troubleshooting for applicable DTC.	Go to Step 3.
3	Exhaust system visual inspection Check exhaust system for leak, damage and looseness. Is it in good condition?	Go to Step 4.	Repair or replace defective parts.
4	HO2S circuit check Check signal and ground circuits of HO2S for high resistance referring to step 3 of DTC P0137/P0138 . Are they in good condition?	Go to Step 5.	Repair or replace defective wire harness.
5	A/F sensor signal circuit check Check A/F sensor signal circuit referring to Step 3 of DTC P0131/P0132/P0134/P2195/P2196/P2237 . Are they in good condition?	Go to Step 6.	Repair or replace defective wire.
6	Replace exhaust manifold (built in TWC), see EXHAUST MANIFOLD REMOVAL AND INSTALLATION (J24B) . Perform DTC confirmation procedure. Is DTC P0420 still detected?	Go to Step 7.	End.
7	Replace HO2S, see A/F (AIR FUEL RATIO) SENSOR, HO2S (HEATED OXYGEN SENSOR) REMOVAL AND INSTALLATION . Check DTC. Is DTC P0420 still detected?	Go to Step 8.	End.
8	Replace A/F sensor, see A/F (AIR FUEL RATIO) SENSOR, HO2S (HEATED OXYGEN SENSOR) REMOVAL AND INSTALLATION . Check DTC. Is DTC P0420 still detected?	Substitute a known-good ECM and recheck.	End.