When is the post-cat O2 sensor tested?

[A_User55555]

So normally a Post-Cat O2 sensor voltage should stay constant between .5V and 1V when a cat is healthy.

However, when looking at mode 6 data, The B1S2 O2 sensor also has a lean and rich test.

The lean test checks if the sensor can read lean between 0V and 0.1V
The rich test checks if the sensor can read rich, between 0.7V to 0.8V

If the O2 voltage doesn't normally reach this range, When is this test run? Is it during startup when the cat is warming up?

 

[brianl703]

On Ford vehicles as I recall it is done at highway speeds after the engine is fully warmed up.

The ECU drives the engine rich, waits a certain amount of time for the rear O2 to read rich (if it doesn't it's a FAIL), and then drives the engine lean and waits a certain amount of time for the rear O2 to read lean (if it doesn't it's a FAIL).

 

[Jetronic]

They could do it every time you lift the throttle.

 

[Timmastertech]

Any answer to this is going to be a generalization because every manufacturer has unique operating strategies (the rear sensor operating range of .5-1 on a healthy cat isnt universally true either). In general terms, rich test is done during power enrichment. When entering power enrichment, rear sensor voltage should go over 900mv because as soon as the feed gas into the cat went rich the cat stopped working and the rear sensor should respond as such. Lean switch testing is generally tested on decel fuel cut which is also when some manufacturers test the cat. Again this is quite general because every OEM and even some variance vehicle to vehicle