Focus Fanatics - View Single Post - Diagnosing P0420 ?
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Old 02-16-2014, 10:22 AM   #5
sykozis
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If the downstream O2 sensor is still switching, it won't produce an O2 code because the PCM doesn't see a fault in the O2 sensor. Instead, it determines the fault to be with the catalytic converter.

As BC_ZX3 said, if you have the ability to do so, monitor the switching of the upstream and downstream O2 sensors.

If they appear to be switching at a relatively similar rate, it's most likely you have either a faulty catalytic converter or faulty downstream O2 sensor. More often than not, replacing the downstream O2 sensor will fix the issue.
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