How can I differentiate if the problem is catalytic converter or the oxygen sensor
P0420 Catalytic below threshold Bank 1
The best way is to use a scan tool and see what the 02 sensor are doing, In order to have a working catalytic convertor you need a proper working 02 sensor, The front 02 sensor or the upstream 02 is the engine management sensor , the rear 02 sensor or the downstream 02 is the catalytic monitor sensor
Here is a typical catalytic convertor, in order for the catalytic convertor to function properly it would need to go from lean to rich in order to work properly
In order to maintain a lean to rich condition 02 sensor are used. The engine management is designed for a functioning 02 sensor,OBD II is designed for the use of a front 02 sensor which is the engine management sensor, and the use of a rear 02 sensor or the downstream 02 sensor to monitor the catalytic convertor.
Here the catalytic convertor is removed, what is in place is a front 02 sensor, this would maintain a lean to rich fuel condition , or what is known as fuel control. where obd II is designed to maintain a fuel ratio of 14.7 to 1.
On a scan tool that has a graphing, you can see what the front 02 sensor is doing, This would be considered a engine that is in fuel control.
Here the rear 02 sensor , the rule is you do not want it to cycle the same as the front 02 sensor, the second rule is it stays above 0.650 volts. which this is doing, yes it does go below 0.650 volts but not to long, this is a good working catalytic convertor.
As the engine is idling and catalytic is warming up, you can see it go less than 0.650 volts. This would be a good catalytic convertor.
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