Okay, we all know the only purpose the downstream O2 sensor has in life is to monitor the cat. It tells the PCM whether the converter is working or not.

The PCM compares the switching rate of the downstream sensor with the upstream sensor. A properly working converter stores a lot of oxygen, meaning the downstream O2 should live a relatively boring life compared to the upstream where there's a lot of stuff going on with combustion. The reading should stay fairly steady and consistent compared to all the switching the upstream sensor does to maintain the correct air/fuel mixture as the engine is running.

As the converter ages, or gets damaged by contamination (coolant, fuel, etc.), it's oxygen storing capability deteriorates. The downstream O2 detects this, and the reading changes - remember, the PCM knows that the downstream is only supposed to operate within "x" range, with only "y" amount of switching. Once the reading exceeds this amount (generally, once the downstream O2's switching pattern begins to mimic the upstream), the PCM turns the check engine light on and says that the something is wrong with the converter.

Now, there are specific downstream O2 sensor fault codes. But in terms of cat efficiency codes, they shouldn't be set by a faulty O2 sensor. The only way those codes should come up is when the switching rates and voltage readings of the downstream O2 exceed a specific limit according to the PCM. In other words, with the limits set on those sensors, they need to be reading and working correctly for the light to come on with P0420/0430. If they aren't working properly, you're more likely to get an actual code listing a specific fault for that sensor (voltage too high/too low, heater circuit failure, etc.).

It's kinda like the parts stores telling folks they need O2 sensors, when they have a rotted out PCV elbow causing a vacuum leak and a lean condition. In the event of a lean condition, the upstream O2 sensors are doing their job by reporting that there's more oxygen than fuel in the exhaust gas, and that they and the short and long term fuel trims can't compensate for it. Likewise, in the event of a failing/faulty cat, the downstream O2s are doing their job by reporting to the PCM that the oxygen storing capability of the converters is nill, and they do that by switching at similar rates to the upstream.

 

WOW !

Awesome explanation. Thanks for the info and education. I am actually saving it to a favorites folder, (called auto-repair). I have some great info in there and that write up will join it.

Thanks Again' Quintin

TJKinCNY

 

All the other codes have stopped, and it went code freea few days, then she started getting the P0430 Cat Efficiency Below Threshold (bank 2) again. Maybe the dealer will diagnose properly this time and swap the Cat.