Oxy sensors How Many?

[Martin Burnard]

Yep as it says.

A3 1.6 2000 (x) appears to have 2 oxy sensors. 1 high up on the manifold and 1 lower down just before the Cat.

The higher one has been disconnected. I assume a second hand cat has been fitted at some stage.

Should it only have one?

Reason for asking is that the emmissions are too high on the CO2 for the MOT.

[Martin Burnard]

Bump Bump.

[A3_FSI_2.0]

IIRC, my old 1998 1.8 sport had 1, after the CAT. Not sure about the newer 8L model . Do you mean that there is a plug not connected to anything or has the downpipe/CAT not got a hole for a sensor etc?

Could be worse, my current A3 has 5 that I have seen!

[ed'sA3-1.8THK]

martin,

the original series A3 (ending late '98) before "refresh" had narrow band single O2 sensor - the revised eu4 series '99 - '04/late - had wide band dual O2 sensors

the wide band sensing locates (1) O2 sensor upstream of cat and the 2nd O2 sensor downstream after cat

the NA 1.6 engine should be equipped with (2) sensors with cat close coupled to exhaust manifold, so 1st sensor should be located near outlet flange of cast iron manifold

you very likely must restore oem/factory O2 sensors AND change spark plugs to pass MOT (plugs have been kept wet by excess fuel due to failed O2 signals)

after this repair you should have better fuel mileage as reward...

change BOTH sensors to obtain correct wide band readings - go to bosch parts agent for replacement sensors - sorry but both must be changed

and than clear ALL error codes and cycle emissions system for correct fuel/air ratio - f/a is what O2 sensors ultimately determine via feedback signals

[Martin Burnard]

Thanks all.

I had been thrown a curve ball by the fact that the car has had a second hand cat fitted at some stage, the Oxy sensor closest to the cat was connected, the one closest to the manifold was not. wiring present only allowed connection of 1 Oxy sensor.

I have replaced the sensor closest to the manifold (previously not connected) and blanked the one by the cat. My local audi agent suspects that the readings would be wrong if the wrong hole was used as temperature would be different.

Will run emmissions check, and see from there.

Audi list 17 options for A3 oxy sensors!

[ed'sA3-1.8THK]

Martin,

there should be an engine vin + emissions id plate in engine bay @ right front wheel sheet metal behind radiator coolant reservoir

if EU4 compliant, wide band sensing, (2) O2 sensors (upstream / dpwnstream cat core)

if EU3 compliant, narrow band sensing, (1) O2 sensor

it is on this basis MOT tailpipe exhaust stream analysis is performed and not simply visual inspection of underside of chassis components

engine ecu different model - different mapping values - and air injection pump added to maintain ignition temp of cat core when engine f/a values requested "richer" mixture, but eng rpm/ex stream velocity low & cat core temp would be dampened/w/reduced reaction rate

the various years/engine models components can't really be jumbled together as entirely different cat cores used eu3/eu4/ulev exhaust downpipe/ex systems 1.6/1.8 5V engines

the later cat core ignite & achieve reaction temp range far more rapidly than eu3 cat cores - breakers good suggestions not withstanding...

I would confirm ecu printout (at audi agent) with id plate in engine bay AND engine serial number to be certain of presence of all original parts AND to correctly id required cat core "type" - aftermarket can provide slow ignition / fast ignition cores if this is specified

using later fast ignition core on early car will not necessarily work as O2 content of exhaust stream mass must be supplemented with additional injected air mass per mapping value requests

just something to consider...

[Martin Burnard]

Life is never as simple as it should be!