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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 
  1. Check DTC.

Is there DTC(s) other than P0420? 		 Go to troubleshooting for applicable DTC. Go to Step 3.
3  Exhaust system visual inspection 
  1. 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 
  1. 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 
  1. 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 
  1. Replace exhaust manifold (built in TWC), see EXHAUST MANIFOLD REMOVAL AND INSTALLATION (J24B) .
  2. Perform DTC confirmation procedure.

Is DTC P0420 still detected? 		 Go to Step 7. End.
7 
  1. Replace HO2S, see A/F (AIR FUEL RATIO) SENSOR, HO2S (HEATED OXYGEN SENSOR) REMOVAL AND INSTALLATION .
  2. Check DTC.

Is DTC P0420 still detected? 		 Go to Step 8. End.
8 
  1. Replace A/F sensor, see A/F (AIR FUEL RATIO) SENSOR, HO2S (HEATED OXYGEN SENSOR) REMOVAL AND INSTALLATION .
  2. Check DTC.

Is DTC P0420 still detected? 		 Substitute a known-good ECM and recheck. End.
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