Diagnosing an O2 sensor in the car

As I've mentioned elsewhere on the forum, I'm quite pleased with my Volvo V50 overall, but the fuel economy leaves a good deal to be desired. OK, it's got a number of things working against it - 200k miles, AWD, automatic - and I know the Volvo whiteblock isn't known for being especially frugal, but I would like to at least get as much out of it as I can. Records on this car are incomplete, so I don't know everything that was done over the last 17 years. I did the spark plugs recently, as they were of unknown vintage, but it didn't make much difference in fuel consumption (it did improve the smoothness of the engine, however). The car got a new air filter when I bought it. The only other wear item I can think of that might be a factor is the upstream O2 sensor. My knowledge of these goes back far enough when there were two wires and you just clipped on a digital multimeter to make sure it was switching quickly enough in the right range. Now there are lots of wires and diagnostics available via scanner, which I have.

My question is how do I make sense of whatever the scanner is telling me? Am I just looking for switching, and if so, in what range? There is no CEL, and the only stored code I've encountered is turbo boost pressure too low, which only seems to set occasionally and is probably related to my slight exhaust gasket leak, though it could also be a sensor issue; it must be fairly minor, as performance seems fine. I know if the O2 sensor failed I'd get a CEL, but what does the process of failing look like? In other words, is there a way to tell if the sensor is negatively affecting fuel economy but is still good enough to be in the acceptable range?

Front sensor on those is a wide band air fuel ratio sensor.  In general, Volvo controls to 1 volt, so when driving, it should be pretty stable around that,  unless you coast, in gear, throttle closed, and it will climb to 5 volts.  The rear sensor is a traditional oxygen sensor, but being behind the cat, it won't tell you a whole lot.  

Depending on the scan tool, you may see traditional fuel trims, or perhaps a correction number.  If you are adding more than a few percentage points of fuel, in all driving conditions, it might be worth changing the sensor.  Adding at idle, look for a vacuum leak.  

Wideband oxygen sensors do drift over time.  At 200k it may not be a bad idea to replace both sensors.

Volvo, like most manufacturers, does use the rear sensor for light trim duties, even if they don't say that they do.  The point of the sensors is to keep the converter happy, after all.

Sitting at a gas station munching on a prefab sandwich, here is where I am at not even trying:

Should hit 500k km this week (310685.6 mi)

OK, I did my mixed test loop with the scanner (Launch 919E) hooked up and recording the O2 sensors readings (and boost pressure, just out of curiosity). Reviewing the readings, the upstream sensor is reading very close to 1v except when coasting downhill, when it is peaking out at close to 16v! That seems far enough from the 5v mentioned to be concerning. The downstream sensor is reading between ~0v and -0.75v, which seems more reasonable. Both seem to be responding rapidly.

Now, just to add to the questions, the specific code I mentioned in the first post is ECM-603D, Turbocharger (TC) Control System. Signal Too Low. The boost pressure during my test drive maxed at ~1250hPa, which seems to equal about 4 pounds of boost. It was my understanding that the T5 engine in the P1 is supposed to boost to roughly 8psi, so again, it seems concerning.

Now I'm wondering if the issue with the turbo boost signal is contributing to the fuel consumption issue.

Edit: For reference, I'm seeing maybe 23mpg in highway-biased driving if I'm conservative - nowhere near what you're getting, Pete.

The TGV (boost solenoid) probably failed. 4psi is unregulated wastegate pressure, the TGV actively bleeds pressure off to increase boost.  Very common.  This will only reduce fuel economy if you are downshifting a lot to make up for the lack of midrange torque.

That failed on my S40 (1.9t/Renix not five cylinder/Motronic) I tried a MAC valve but for reasons I forget it wasn't happy with it, so I bought a Turbosmart manual controller and played with that. I ran into detonation over 9psi so I kind of left it there.

Fuel economy is also the basics... are the brakes binding, what is the alignment like, etc.  I also like to run about 15%ish more pressure in the front tires than the rear.  This helps highway economy a fair bit, as if the front tires have a shorter rolling radius, the rear coupler is always fighting to supply power to the rear wheels, which adds drivetrain resistance.  I came up with this number by shrugging, remembering that my Subaru could be shifted in and out of 4wd with no resistance with pressures of 44/35, and shrugging some more   I think I set my pressures to 40/35 when I mounted the new hoops, but I also kind of never check them.

The lean limit being so high should not be a big deal- but during steady driving, what is the rear one at- .75v?  
 

Not only is the rear sensor used for trim, it's also used to correct the WB, as the NB is really accurate at knowing stoich. It also ages really slowly, so even at that mileage, I would trust it. It probably isn't spending much time less than .5 unless you are coasting and the fuel is off. 

Oh yeah... I have a well aged converter that got poisoned by all the coolant that I was processing a few years ago, so I have an "adapter elbow" on the downstream sensor.  The elbow plugs with carbon over time and the computer throws a "insufficient switching" code that I can fix by heavy application of carb cleaner to the sensor and elbow.  Fuel economy drops markedly when this happens.

I HAVE a new converter, a real shiny ipd unit... and between the elbow design and the size of the cat, it is more restrictive than stock (boost drops 2psi before it relearns the TGV settings), and the downstream sensor hits the driveshaft, so if I have to run an adaptor elbow anyway I may as well use the OE downpipe.  Also the flex section is way too short and it makes the car buzz sometimes.

BTW, I don't think what you are reading for the front sensor is voltage. It looks more like lambda, 1 is stoich, higher is lean, lower is rich. Assuming gasoline, it's a/f divided by 14.6. But it doesn't care about the fuel 1 is stoich always. And going to 16 is just a number that is a limit to be air. 

Here's a representative sample of what the data looks like:

Green is the upstream, blue is the downstream. Sorry the scale is so hard to read, but the ranges are as noted earlier.

I'm running equal tire pressures F/R right now, but bumping up the fronts 15% is easy enough to try.

Replacing the boost solenoid seems a pretty obvious first step to correct the boost issue, even if it doesn't impact the fuel consumption much or at all.

Edit: Sorry, alfadriver, I just saw your last comment. That could very well be the case.

From what I can tell your mpg is fine. 
 

https://www.fueleconomy.gov/feg/bymodel/2008_Volvo_V50.shtml

In reply to dean1484 :

Volvos do poorly on the EPA test vs real world a lot of the time because Volvo didn't care about trying to ace the test.  That is why their automatics are not maddening.

So, per the discussion above, I replaced the TCV. It's a very fiddly job, with pretty lousy access. I went in from the side after trying and mostly failing to get at it from below. I planned to relocate it to the firewall, which seems a common mod, but there was no way to get some of the hoses off the turbo, so I gave up and left it in the stock location. All the hoses seemed pliable and in good shape, so fingers crossed it holds up.

On the plus side, on the same test loop I saw a peak boost of 1650hPa, which is just over 9psi, so obviously the TCV was an issue. The car absolutely scoots now, which is going to make it harder to drive in a way that focuses on fuel economy.

While I was down there, I threw on a quick and dirty Home Depot lip spoiler, just to gain any little aero advantage it might provide. Fits easily and the color matches the stock trim almost perfectly.