Car is 1996 A4 2.8 auto 2 valve AFC.

Car gets 12 mpg (90% city driving).

I noticed that white smoke from the tail pipe, not just during warm up, but also when oil and water show operating temperature.

I have engine code (when erased, may not come back right away, but sooner or later it does). IIRC, this is driver side downstream sensor, brown plug):

1 Fault Found:
16540 - Oxygen (Lambda) Sensor B2 S2
P0156 - 92-00 - Malfunction in Circuit
Readiness: N/A

I was always under the impression that the downstream O2 is NOT influencing the engine control, purely 'checking' the status of the catalytic converter (only the upstream ones are used to check air-fuel ratio, etc.)

a) Is low gas mileage related to O2 code?
b) Is white smoke indicative of running way too rich all the time?
c) Is white smoke, low gas mileage, O2 code not ALL related (but 2 of the 3)?

 

yes !! o2 reads air fuel mixture at the pipe and helps car to adjust . change out that o2 clean up your throttle body and use good quality gas 93 only (im a big fan of mobil gas)

 

white smoke usually indicates water, aka head gasket issues, warped head, look at the oil on the dipstick for signs of water-antifreeze mix

 

Yeah, that's my first thought when white smoke. But NOT enough (big plumes) smoke as I have experienced on typical head gasket cases.

Changed oil yesterday: not milky, watery at all.

O2: are you sure you are talking about the DOWNSTEAM one?
Keep in mind: this one is often deleted (dummy resistor), tuned out (installation of downpipe, piggies, cat delete, etc.). If they plaid an active role: engine would never get a usable signal in those cases.

Can someone confirm?

BTW oil: Since oil is of highest importance for my turboed cars (not so much this one), and among those the particularly the 1.8T AEB (sludge!!!), I dug deep into the 'best oil' discussion.

Conclusion: they changed the definition of 'full synthetic' a few years back, and now a lot of oil can call themselves 'full synthetic'. There are only a few 'true full synthetic' left (who still meet the old, more stringent requirements).

The 2 I find in my area are:

Mobil 1 "European Formula" (on the front) (0W-40)
Castrol "European Formula" (on the front) and "Made in Germany" on the back (also often referred to as GC or Germany Castrol). the 0W-30 behaves like a 0W40.

When they are on the shelf, I pick them up at Autozone, O'Reilley, Advanced Auto Parts if they are on sale (normal price $8.59 or so).

 

yes BP aka castrol was the slimeballs that fought that law thru, same people that polute the gulf with big oil leak!!! i like the amsoil full synthetic or redline for even more heat protection since its ester based oil$$