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Discussion Starter #1
I've got a 2012 Cruze Eco/6 spd that has 215k miles on it. My daily commute is 100 miles round trip, and it's my daily driver, since it normally gets such great fuel mileage -- usually around 42 mpg, except in summer when I've gotten as high as 48.9 mpg for a 500 mile average. (These are according to the instrument panel readings; my actual mpg's tend to be lower when I do the math, as I do at each and every fill-up.)

Recently (at around 212k miles) my Cruze Eco started getting horrendous fuel mileage and the CEL illuminated. Instead of my normal 42 mpg average over 500 miles my economy dropped all the way down to 24 mpg after about 500 miles. While my "instantaneous" mpg are often 48-50 mpg under light cruising, when the problem set in, I could rarely get it to show better than 32 mpg, instead, unless I was literally coasting downhill with my foot off the accelerator. The car essentially ran ok ... a bit sluggish going up hills, but otherwise not bad.

Since I noticed that it still got normal (50 mpg-ish) instantaneous mileage until it warmed up and went into closed-loop mode, it obviously suggested an issue with the oxygen sensors, and when I scanned for codes (which I actually did pretty much immediately) there were several codes set, including:
  • P0171 (Bank 1 too lean)
  • P0133 O2 slow response
  • P015B O2 delayed response
  • P2096 Post cat fuel trim too lean
In addition, my scanner (which displays live data) showed crazy high short- and long-term fuel trim values. >40% for both, unless letting the engine spin down and having my foot completely off the throttle.

At first, I tried cleaning the O2 sensors, which were covered in soot. That did nothing.

So, I replaced both O2 sensors, but made the mistake of using cheapo local parts store parts since those were all that I could get on short notice, and they didn't fix the problem.

I did smoke tests (from the air inlet duct inlet ahead of the MAF; from the power brake booster vacuum hose source on the intake; and into the tailpipe) and found no leaks.

I checked the voltages to and from the O2 sensors. The readings were normal.

I checked spark plugs. Normal.

Air filter? Clean.

I checked the fuel vapor solenoid. Fine.

Even though the problem characteristics and codes were pretty clear that the problem was with the O2 sensors, I replaced the valve cover in case the integrated PCV valve was allowing too much air to be introduced, thus causing the lean condition. That didn't fix it.

I even borrowed MAF and MAP sensors from another well-running Eco to verify that those weren't somehow affecting things. No improvement.

And, figuring that my old injectors had 212k miles on them, I replaced them with new AC Delco units. No difference.

After a couple weeks of trying all that and probably more, I broke down and took it to the local Chevy dealer, who immediately witnessed the symptoms. They kept the car for 3 days doing all sorts of tests -- many of which were the same things I'd done -- but found nothing that explained the problem. Ultimately, they replaced the upstream O2 sensor with a new AC Delco unit. I told them when I picked it up that I was skeptical that that would truly fix the problem, since I'd tried new O2 sensors (albeit with cheapo parts store ones), but, indeed, the car seemed to run fine and the fuel economy came back after they put in the new O2 sensor.

But about 3 weeks later, the problem returned. It happened to return right after I stopped to fill up (Sunoco 93 octane premium, as usual) during a heavy downpour. Before stopping for gas, it had been running fine and getting 42 mpg or so. But after filling up, when I went to pull away from the station, the car seemed to be down on power, so I stepped on it a bit to get up to speed ... and immediately noticed that the fuel economy didn't recover after I settled into a normal 65-mph cruising speed on the highway.

I took it back to the dealer. They did 2 more days of tests by a 25-year Master Mechanic. But found no reason for the problem ... though they could clearly see that the problem had returned. They submitted a request to GM for diagnostic assistance, but to date I have heard nothing back ... nor do I really expect to. I mean, let's be real: it's a 215k mile almost-10-years-old car. Neither the dealer nor GM really care about it. They'd both rather sell me another vehicle to replace it.

So I got the car back, then I replaced both O2 sensors with new AC Delco units, and it's been running fine again for about 3 weeks. I'm back to getting 43 mpg (over a 500 mile average). It actually seems to be getting better fuel economy than it ever had in the past 100,000 miles since I bought it. But I have zero confidence that it will stay running properly. I'm expecting these O2 sensors (which are nearing a month old now) to fail any day, like the last ones. And I'm afraid to ever accelerate at any pace faster than a little old lady would, for fear that maybe stepping on it when I left the gas station that day was somehow what triggered the problem (though I only stepped on it because the engine was way down on power already ... so I think stepping on it was really after-the-fact).

Anyway ... anyone ever heard of anything like this? More importantly: got any idea how to fix it?

Thanks for any suggestions or thoughts or even links to related topics (I tried searching for similar circumstances, but didn't find any ... my apologies if I missed a similar thread).
 

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Discussion Starter #3
So far, so good. I think they've been in about a month now ... so roughly 2000 miles.

Amazingly, the car seems to be getting even better fuel economy than it has in the past, especially given how cold it's been lately. Just today, while on a 100-mile trip, my 500-mile average inched up to 43.8 mpg (according to the instrument panel's calculations ... I'm sure it'll be closer to 41 mpg when I do the math at my next fill-up).

Unfortunately, since I don't know what caused the previous O2 sensors to fail, I have zero faith that these will keep working well, so every time I start the car and set off on a journey, I get that anxious feeling: Is it going to continue to work well? Or will one or the other or both suddenly die on this trip?

And even though the car runs "ok" when the sensor(s) fail, I've resorted to carrying replacements for each O2 sensor (and the wrench to change them) in the car for what feels like an inevitable failure sometime in the not-too-distant future.

It's a shame, because I really love the car ... I just hate not being able to trust that it will continue to operate properly.
 

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Ok, thanks for the update. I'm putting in a new pre-cat sensor tomorrow, but mine has been malfunctioning since I bought it 30k kms ago, and who knows how long before that. It still worked 90% of the time but that 10% is enough to completely mess up the fuel trims.
If yours fails again, it will be something causing it rather than just bad parts. I wouldn't know where to start myself, other than something in the exhaust that's not supposed to be there. Or being blocked on the external end of the sensor. They have a hollow part to allow outside oxygen in as a comparison to the exhaust oxygen...
 
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