# Bad MPG - Possibly O2 Sensors or brake drag



## wasney (Mar 3, 2015)

After some research I believe I have narrowed down the 2 possible causes of my less than average MPG. 

2013 LT 1.4L

1. How do I check for brake drag? Just jack up the front end, put it in neutral, and spin the front tires?

2. How can I for sure know if the O2 Sensors are a culprit? I am a little over 80,000 miles and read some people change them at 100k. No check engine light at this time.

3. I keep my tires at the manufacturer recommendations. I read in the MPG thread from Xtreme that putting them to the max can give better gas mileage. Is it worth putting the to the Max PSI?

Thanks guys!


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## obermd (Mar 3, 2012)

You can check for brake drag by taking your car out for a drive and then feeling the brake temperatures. Brake drag will be hot to the touch.

Not sure on the O2 sensors but I suspect if they're bad you'll get a code.

Run your tires anywhere between the door placard and the max sidewall. Measure PSI cold. Find the sweet spot for performance & comfort for you and keep them there. The overall change in MPG isn't that much going from door placard to max sidewall PSI.


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## wasney (Mar 3, 2015)

obermd said:


> You can check for brake drag by taking your car out for a drive and then feeling the brake temperatures. Brake drag will be hot to the touch.
> 
> Not sure on the O2 sensors but I suspect if they're bad you'll get a code.
> 
> Run your tires anywhere between the door placard and the max sidewall. Measure PSI cold. Find the sweet spot for performance & comfort for you and keep them there. The overall change in MPG isn't that much going from door placard to max sidewall PSI.


Will check for heat. I'll probably just do 40 psi for the tires then and see if there's a difference.

Thanks!


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## TDCruze (Sep 26, 2014)

O2 sensors can get "lazy" over time and this can result in the engine running a bit richer and lower your gas mileage. As the O2 sensor ages they begin to react slower than when new, but if the readings are still within PCM allowable range of operation you will not get a CEL. Without a gauge or scanner to see the O2 sensor output voltage it is hard to tell for sure if the O2 sensor is a possible issue.


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## wasney (Mar 3, 2015)

TDCruze said:


> O2 sensors can get "lazy" over time and this can result in the engine running a bit richer and lower your gas mileage. As the O2 sensor ages they begin to react slower than when new, but if the readings are still within PCM allowable range of operation you will not get a CEL. Without a gauge or scanner to see the O2 sensor output voltage it is hard to tell for sure if the O2 sensor is a possible issue.


Well I ruled out the brakes. After driving for about 20 mins I was able to touch the rotor with my bare hand and it wasn't hot at all. What kind of thing would I need to check the O2 sensors?


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## ChevyGuy (Dec 13, 2014)

wasney said:


> What kind of thing would I need to check the O2 sensors?


You'd need a scanner or a scope that can watch the output of the O2 sensor so you can see how fast they are cycling. But without a number to compare it with, I don't know as it would do much good.

I'm pretty sure the O2 sensor is cheaper than the tool needed to test it.


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## wasney (Mar 3, 2015)

ChevyGuy said:


> You'd need a scanner or a scope that can watch the output of the O2 sensor so you can see how fast they are cycling. But without a number to compare it with, I don't know as it would do much good.
> 
> I'm pretty sure the O2 sensor is cheaper than the tool needed to test it.


I might see if my friends dads shop has the tool to check then before I spend money on them. I am sure someone here can find the specs for a "normal" reading. Thanks!


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## ssnscruzeeco2015 (Dec 29, 2014)

if the O2 sensor is bad enough it will flag codes including P0133 which flags slow response. My sensor failed with 132 and 134 if I recall, it cost around 100 dollars CAD to replace.

http://www.cruzetalk.com/forum/9-general-discussion/185498-o2-sensor-failure-21-months-83000km.html


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## wasney (Mar 3, 2015)

ssnscruzeeco2015 said:


> if the O2 sensor is bad enough it will flag codes including P0133 which flags slow response. My sensor failed with 132 and 134 if I recall, it cost around 100 dollars CAD to replace.
> 
> http://www.cruzetalk.com/forum/9-general-discussion/185498-o2-sensor-failure-21-months-83000km.html


That is what everyone else is saying. It seems like I have always had worse MPG then others with the same car driving in mostly city. I have ruled out the common factors and have read many people say they fail overtime and you lose MPG and eventually will throw a code. It will cost me about $80 USD to do it. But I don't want to do it if there is no need.

Does anyone know what the "regular" readings are for O2 Sensors?


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## ssnscruzeeco2015 (Dec 29, 2014)

what do you have access to monitor your ECU? I have a Bluetooth OBD2 adapter and Torque app on my phone? You can actually measure and monitor the output voltage of the O2 sensor there.


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## jblackburn (Apr 14, 2012)

wasney said:


> That is what everyone else is saying. It seems like I have always had worse MPG then others with the same car driving in mostly city. I have ruled out the common factors and have read many people say they fail overtime and you lose MPG and eventually will throw a code. It will cost me about $80 USD to do it. But I don't want to do it if there is no need.
> 
> Does anyone know what the "regular" readings are for O2 Sensors?


See what your fuel trims look like. Anything over +/- 6 or so may point to some sort of vacuum/air/exhaust leak that is actually tricking the sensors into using more fuel than other comparable cars.


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## StormShepherd (Jan 29, 2017)

ChevyGuy said:


> You'd need a scanner or a scope that can watch the output of the O2 sensor so you can see how fast they are cycling. But without a number to compare it with, I don't know as it would do much good.
> 
> I'm pretty sure the O2 sensor is cheaper than the tool needed to test it.


Typically, O2 sensors send a signal between 200 and 850 mV, though some brands do weird stuff, such as Chrysler who have a 5 volt diagnostic voltage applied to the circuit. Bad O2 sensors typically favor one side or the other of 500mV, or will hang in a specific voltage range all the time. Last time I saw a bad voltage on an O2 it favored 200mV, and wouldn't jump from it more than 50 mV, as an example. It should bounce back and forth from 400 to 800 on closed loop.


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