DTC Troubleshooting

DIAGNOSTIC INFORMATION AND PROCEDURES REFERENCE CHART

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 .
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 according to step 3 under 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 according to Step 3 under DTC P0131/P0132/P0134/P2237 . Are they in good condition?	Go to Step 6.	Repair or replace defective wire.
6	Replace exhaust manifold (built in TWC). Refer to EXHAUST MANIFOLD REMOVAL AND INSTALLATION . Perform DTC confirmation procedure. Is DTC P0420 still detected?	Go to Step 7.	End.
7	Replace HO2S. Refer to HEATED OXYGEN SENSOR (HO2S) REMOVAL AND INSTALLATION . Check DTC. Is DTC P0420 still detected?	Go to Step 8.	End.
8	Replace A/F sensor. Refer to AIR FUEL RATIO (A/F) SENSOR REMOVAL AND INSTALLATION . Check DTC. Is DTC P0420 still detected?	Substitute a known-good ECM and recheck DTC	End.