bad fuel mileage question on a 100000 mile mini question

[Socalcoopster]

Hey all

I have a 2005 Mini Coopers S. Its long been out of warranty
and Im ready to do some work. I have 104000 original miles by me!
I use to average 28-30mpg and now only getting 23-25mpg and My driving habits have not changed.

I will probably replace the spark plugs (have ngk iridium ix gone 30000 miles on them ) , cables (original never replaced) , clean the air filter ( I have
a rouge CAI) drive belt ( I have a 15% reduction pully)oil and filter,cabin filter. Now which brings me to the bad fuel mileage question,
I read somewhere that the bad fuel mileage could be the o2 sensor, anybody
else read or know of this? I currently do not have a check engine light, but had one before for "engine too lean " but it has gone away.

Any other things I should replace/or address?

thanks

[Socalcoopster]

*BUMP* maybe this is in the wrong forum moderator??
anyhow after some digging in other sites, I think the bad mpg
is in fact a bad O2 sensor, all the symptoms of one has happened.

[XRV848]

A failed oxygen sensor should produce a fault code and turn on the malfunction indicator light (often called, confusingly, the “service engine soon” light). Marginal operation of the sensor could affect mileage and not cause the light to be turned on, but there may be a pending code set and a decent scan tool should show this status.
A good oxygen sensor produces an oscillating waveform that toggles between 0.1V to 0.9V. The sensor indicates between 0.1V for lean mixture and 0.9V for rich air to fuel mixture. The OBD2 tests how fast or slow the oxygen sensor transitions between lean and rich mixture levels (usually referred to as “crossings” as the voltage crosses the nominal mixture value) and compares the response time of the oxygen sensor with known good values. More comprehensive OBD2 scan tools will also display the oxygen sensor output waveform as seen using an oscilloscope, or a high impedance volt meter across the sensor will show the activity.
The oxygen sensor compares the oxygen levels in the exhaust with the outside oxygen level. If the sensor is coated with mud or undercoating then this will cause it to mis-read.
Note that I am referring to the sensor before the catalytic converter. The post-cat sensor is there to monitor the effectiveness of the catalytic converter and its failure will not affect the engine operation.

[Socalcoopster]

I did have a "service engine soon" light appear

the fault code reader gave me this :

P2096 Post catalytic converter fuel trim , bank 1 - too lean

the light would come on intermitenly off and on, I would clear
it and re check and it gave me the same error code, after about 10,000miles
its has gone off and not re-appeared. To be honest I havent check
to see if theres a error code still there.

Again the fule mileage dropped right after this was starting to happen
so if the o2 sensor stops working completly does that trigger another
"service " light?

[XRV848]

The P2096 code relates to the post catalytic converter oxygen sensor and shouldn't affect the milage. I believe that it is set after the sensor reaches its operating temperature when the system runs a monitor (test) to determine if the post-cat sensor is working. The P2096 code is a lean condition and this should not result in reduced milage, unless the system enriches the mixture to correct the condition! That would be the tail wagging the dog as the pre-cat oxygen sensor is the one that should control the fuel/air mixture.
I would scan the system and see if there are any fault codes or pending codes stored Monitor the oxygen sensor output if you can and see how it's operating. It is possible that the pre-cat sensor is faulty, but there are many other faults that cause low milage. I think too many oxygen sensors are changed unnecessarily and often see the "service engine light= replace oxygen sensor" advice given without any diagnostic work.

[Socalcoopster]

thanks for the info, ill run the code reader again. Like I said
the o2 sensor would be a last ditch effort after cleaning the air filter
, replacing the plugs, maybe take a look at the injectors.