How to Diagnose a P0420 Code

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• Locate the suspect converter

• Check for exhaust leaks

• Warm up the converter

• Graph O₂ sensors

• Interpret waveforms

• Confirm with O₂ sensor test

• Replace and confirm

Locate the suspect catalytic converter

1. Locate the suspect catalytic converter to be tested – ↗ 0:50

Check for exhaust leaks

2. Visually inspect for cracked or rusted-through components. With the engine cold, start the vehicle and listen for leaks—pay attention to connections and flex pipes – ↗ 1:44

3. If a hard-to-find leak is present, pinpoint it using soapy water and a shop vac – ↗ 2:42

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Warm up the converter

4. Bring the engine to operating temperature so the catalytic converter is hot enough to function properly – ↗ 5:04

Graph O₂ sensors

5. With a scan tool or lab scope, graph the upstream and downstream O₂ (or A/F) sensor signals – ↗ 5:28

Interpret waveforms

6. Monitor signals at idle and ~2000 rpm. A healthy converter shows a relatively steady downstream O₂ trace; a failed converter shows downstream activity mirroring the upstream – ↗ 5:31

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Confirm with O₂ sensor test

7. Test the O₂ sensor to confirm results – ↗ 6:15
   

Replace and confirm

8. If testing confirms a failed catalytic converter, replace it with a quality part – ↗ 7:01

9. After replacement, confirm proper operation of the new converter – ↗ 9:22