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2008 cts
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27 Posts
Discussion Starter #1
Need help! My mixed mpg has historically been about 16mpg, which I think is low. Lately it’s dipped to 14mpg mixed use and now it’s suddenly going lower. I have a P420 code check engine light for inefficient cat, but it comes on and off. I have new spark plugs, air filter and ran gas treatments though it. Power seems fine except for a little hesitation when stomping on pedal.

What could explain the 20-40% fuel efficiency reduction? A bad cat? Bad o2 sensors? I thought cats where designed to survive the life of the car.

Help! Thanks for the input.
 

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2008 CTS Base Manual Tran
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553 Posts
Re: Help: 2008 cts 3.6di rwd

A DI RWD should be getting 20-21 mixed driving. I am getting 20 from my non-DI. I have not heard of that low of mileage on this site..are you independently checking it or relying on the computer?
 

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2008 cts
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27 Posts
Discussion Starter #3
Directly checking when I fill up my tank. I used to get 240-260 mpg per tank (which I think is low), but now it’s suddenly about 180-200. Something is definitely off, even though engine appears to run decently smooth. Could a bad 02 sensor cause that much of a fuel drop?
 

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08 CTS DI
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2,301 Posts
The oxygen sensor is the only way the computer knows for sure it has the correct air fuel mix so yes, it can. Some cars are hardly driveable with an oxygen sensor failure.
 

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2009 CTS4 Premium & 2012 SRX4 Luxury
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683 Posts
What could explain the 20-40% fuel efficiency reduction? A bad cat? Bad o2 sensors? I thought cats where designed to survive the life of the car.
They are, but are damaged by burning oil or improper fuel mixture, which makes it very important to fix O2 sensor issues promptly. What was the condition of the old plugs when changed?

I agree your mpg is horribly low. My AWD gets 20-21 mixed, 25 highway.
 

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08 CTS DI
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2,301 Posts
Go to ebay and search for an O2 sensor spacer, it's basically a short extension that moves the O2 sensor a little further away from the exhaust stream. Some motor heads that gut their catalytic converters use them to trick the PCM into thinking the converter is still in place to keep the engine light off. A properly functioning converter will cause a voltage difference between the up stream and down stream O2 sensor. As converter becomes less efficient, that voltage difference decreases into the threshold that identifies the converter as being worn out. The spacer helps increase the voltage difference. It's pretty cheap ~$5 and you can probably find it at a local parts store. If you try it and it solves the problem, you have a bad converter, which is probably the case. I can't say for sure that a bad downstream O2 sensor would suggest a good converter is bad as I don't know what the fault requirements are before the code is set.
 

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2008 cts
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27 Posts
Discussion Starter #9
Thanks all for your help. I’m gonna start with the o2 sensors and go from there. I need a left bank upstream sensor and a left bank downstream sensor. Two questions:

1. What are the part numbers for those sensors so I can order the right ones.

2. Are denso or ngk/t brands ok? They are significantly less than current Delco prices.

Thanks!
 

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2008 CTS AWD
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37 Posts
So to piggy back on this I have an 08 AWD that reliably gets above 20. Last 2 tanks have been 15-16 no different driving going on. Wife is saying feels sluggish also. No CEL on I will drive it tomorrow but any ideas? Wondering if O2 sensors fixed the issue here as sounds pretty much the same.
 

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Go ahead, blame me. Everybody does 🙄
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9,481 Posts
Check for pending DTC's.
 

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'09 CTS DI FE1 Luxury
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1,469 Posts
I get 21 mixed but put it down to the 3.42 rear gear. Lots of mpg particularly 60-80 on the highway (26 mpg). ;)

 

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**2014 CTS4 Sport Wagon-Luxury**
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2,183 Posts
...change to a Winter gasoline formulation???...good luck

Bill
 

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Go ahead, blame me. Everybody does 🙄
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I wish the new site had a line that showed location as the old one did; many people don't even put it in their inner profile.
Sure would help with concerns like this.
 

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'09 CTS DI FE1 Luxury
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1,469 Posts
I would look at the usual causes first like dirty air filter, old plugs, low tire pressure, dirty MAF, throttle body, etc. Bad O2 sensors will set a check engine light but if yours have over 100k miles on them you might consider replacing them as maintenance. There's a good chance that your intake valves are gunked up if DI. Good luck.
 
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**2014 CTS4 Sport Wagon-Luxury**
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CaddyFred...good recommendation...start with the easy stuff and rule that out...car benefits either way (y)

Bill
 

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2011 CTS Coupe FE3, 2003 Thunderbird, Gone 2013 ATS, 02 Deville
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1,673 Posts
Crusher how many miles on this 12 year old car? and what maintenance have you done on it last 5 years?
 
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