I took delivery of my new to me 2014 mk 3.5 1.0T focus (125) at the end of December.  A week ago the engine management warning light came on saying and the garage says it needs a new post cat oxygen sensor.  The car drives absolutely no differently to how it did before, and over the last couple of tankfuls on my commute its return a healthy 45 mpg.  So far so good.

I have a wee bit of a doubt though, I've heard this engine described as 'lively, rev happy' and so on - but I'm not sure I would agree with that.  Bury the throttle and it gets round the rev counter but feels like you are eeking it out.  It does seem to get a second wind at about 5k rpm.

Obviously there is a known fault at the moment, but I always thought that rear oxygen sensors didn't really affect performance as such and were there only to monitor the health of the cat.  So wouldn't expect it to really affect anything.  Its not limited to any revs (limp mode) as far as I can tell.

Are you supposed to hear a whistle or hiss - indications that it does actually have a turbo?  I can't hear anything.

Like I say, it was like this before the light came on - and we'll see soon enough if a new o2 sensor helps.  But wondering if there is something else amiss as well.  

On 17/01/2018 at 10:12 PM, ub7rm said:

I took delivery of my new to me 2014 mk 3.5 1.0T focus (125) at the end of December.  A week ago the engine management warning light came on saying and the garage says it needs a new post cat oxygen sensor.  The car drives absolutely no differently to how it did before, and over the last couple of tankfuls on my commute its return a healthy 45 mpg.  So far so good.

I have a wee bit of a doubt though, I've heard this engine described as 'lively, rev happy' and so on - but I'm not sure I would agree with that.  Bury the throttle and it gets round the rev counter but feels like you are eeking it out.  It does seem to get a second wind at about 5k rpm.

Obviously there is a known fault at the moment, but I always thought that rear oxygen sensors didn't really affect performance as such and were there only to monitor the health of the cat.  So wouldn't expect it to really affect anything.  Its not limited to any revs (limp mode) as far as I can tell.

Are you supposed to hear a whistle or hiss - indications that it does actually have a turbo?  I can't hear anything.

Like I say, it was like this before the light came on - and we'll see soon enough if a new o2 sensor helps.  But wondering if there is something else amiss as well.  

I've driven a few 125 BHP EB MK3.5's and I'd definitely agree with the "rev happy" description, if yours feels more "laboured" then I'd say something isn't right, I'd be interested to know if the O2 sensor makes any difference.

I have never driven an ecoboost so can not comment on how they feel.  But, like you, I thought the post cat oxygen sensor was purely to check the cat is effective and has no other effect on anything at all.  But who knows what they may have done to make cars ever more complcated

So the sensor was swapped out and other than the light is now off there is no difference in how the car drives.  While the car was getting fixed I had a loaner 1.0T and other than feeling a bit crisper with the clutch (newer car) it felt the same.  

 

Suspect I need a remap...

23 hours ago, ub7rm said:

So the sensor was swapped out and other than the light is now off there is no difference in how the car drives.  While the car was getting fixed I had a loaner 1.0T and other than feeling a bit crisper with the clutch (newer car) it felt the same.  

 

Suspect I need a remap...

I think @iantt has his 100 BHP 1.0 EB Focus remapped with very positive results with yours being the 125 BHP and 6 speed box I'd expect you to see a nice bump in performance with a decent remap.