I've got a few problems here.

1: The car is bad on gas (like, unusually bad... 12-13MPG maybe?)
I drove it pretty gently - Has lots of compression and a fresh engine. CEL isn't on, I'm assuming it would come on if the o2 sensor was bad?

2: it doesn't seem to have much power at high RPM - it seems really dead above 6000. Below 6000 it's fantastic! Is this just because it's stock and I've forgotten that they don't pull up high? Been a few years since I've driven a stock T2.

 

could be a clogged cat

 

its an s5, theres your problem

 

A dead O2 sensor would only have a small effect on fuel consumption, <5% unless you're doing all highway driving. There are lots of far more likely causes of high fuel consumption and lack of power. A clogged cat would be my first thought.

 

Doesn't hurt, and it's relatively cheap to replace.
IME, a bad O2 sensor would drop mileage 2 - 3mpg.
A bad or misadjusted TPS would also affect your gas mileage at about the same rate.


Stock exhaust?
Yes, it chokes the power a lot.


-Ted

 

On my car the, with mixed driving conditions, the difference between a new O2 sensor and no sensor at all was less than 5%. I reckon 2-3mpg (10-15%) improvment just from replacing the O2 sensor would be miraculous... The fact that it only works in closed-loop cruising and only trims mixtures slightly means it's overall effect is pretty small.

 

it's BONE stock.

I mentioned O2 sensor because when I traced the wire to make sure it was connected it went to some weirdo white wire that went aaaallll the way over to a hole on the driver's side of the firewall, I haven't managed to trace where it gets to inside or not.

How do I pull the codes on this thing to see if everything's connected?

I just drove it and 1/2 a tank didn't even get me 100 miles! And this is an s5 with the bigass gas tank.

I haven't adjusted the TPS yet either but it seems to be reasonably close to adjustment - at least not "way out"... it does backfire some on high-rpm decel though so maybe it's a part of the problem.

 

If that includes the exhaust then I'd put money on a clogged cat.