Originally Posted by mensajero
The second one sees that the first one is working.
this is not true for the bank1 sensor2 (b1s2). It can't really read what the bank1 sensor1 (b1s1) is doing because the A/F ratio is changed in the catalytic converter.
The ECM checks the bank1 sensor1 (b1s1) two ways.
Running at a certain fuel trim % there is a approximate value that the (b1s1) should be at.
Or if it changes the fuel trim % (+/-) the b1s1 sensor value should change accordingly.
If it is out of range you get a CEL. The ecm will then default to a rich fuel trim (same as cold start as the b1s1 is out of range during cold start but without the CEL )
The ecm uses the b1s2 for for monitoring the catalytic converter (cat) efficiency by comparing b1s1 values with the after cat b1s2 sensor value.
Originally Posted by mesajero
It doesn't matter how far one is from the other in your exhaust system.
I think it matters somewhat , but more based on the cat placement (I could be wrong. I have never read any info on cat placement in relation to its performance but I'm asmuming that they put it there for a reason). The cat should be placed as close to the original spot as possible (to help keep warming time low) with the sensor as close to the factory distance from the cat as possible.