Good evening.
The 420 is set when the rear O2 sensor reading is not steady. If you look at the reading, you need to see around 4.5 volts and it may vary by only.1 or.2 volts either way. When it varies more than that, the sensor starts to act like the front sensor.
The only job of the rear O2 is to monitor the catalytic converter. When the readings are off and the code sets, then the cat is worn out and needs to be replaced. The precious metals inside are no longer doing there job processing the gases from the engine.
Roy
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Thursday, June 28th, 2018 AT 5:10 PM