P0420 is only for the downstream O2 sensor.
A gas cap would trip an EVAP leak detection code.
Bad gas would trip the upstream O2 sensor as well.
You need to watch the datastream to verify the sensor output. It could be an electrical intermittent, PCM fault, sensor performance, or converter malfunction.
You CAN'T find the cause without more diagnosis. Fixing the symptom won't cure the cause.
There are still SRS codes to read for the airbags.
A consumer grade scanner can't access them.
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