O2 sensor question

[Chromed]

Am I correct in thinking the sensors after the cat are there just to tell the ecu the cat is working, and all the engine management data comes from the pre cat sensors? Thanks in advance.

[Mr2Mike]

In general yes, the post cat sensors are to ensure the cat is in place and working effectively.

[anonymous-user]

The downstream sensor is also used for fuel trimming to adjust the catalysts time average lambda bias. However, this trimming is a very small amount, less than ~1% for a normally operating system

[Chromed]

Thanks guys, so is the fuel trimming mentioned in regards to emissions only ? (On the post cat sensor)

Edited by Chromed on Friday 12th July 06:04

[GVK]

If you log both sensors you'll see the pre cat sensor switching during idle/cruise but the post cat sensor (if cat is efficient) will just be a steady voltage. If the voltage trace from post cat sensor mirrors pre cat = cat is fooked!

You get fuelling adaptions called short and long term fuel trim where the ECU monitors the sensors and adjust the fuelling accordingly. Max_Torque will be able to explain those in greater detail and more eloquently than I could! :P

Edited by GVK on Thursday 11th July 19:35

[AER]

short and long term trims are two functions for the upstream sensor only. The downstream sensor is used to trim the lambda biasing - basically an aggregate of how much oxygen the cat stores/sees. That and catalyst monitoring for OBD functions.

Downstream O2 trimming has been responsible for a fair bit of the improvement in emissions capability in "modern times" since a fairly knackered cat can be made to do some impressive things, still delivering close to not very much at all out of the tailpipe.

No driver will ever know whether it is functioning correctly or not, save for the check-engine lamp illuminating.

[anonymous-user]

Cheers Andy, that ^^^ was what i was going to say ;-)

[Chromed]

Many thanks.