P0139 - Bad MPG?
06-04-2012, 12:41 AM
#1
Hexrx
Thread Starter
P0139 - Bad MPG?
So I deleted the cat, and to no surprise a few hundred miles later I get this bad boy CEL code.. "P0139 Rear HO2S circuit slow response ON 2 HO2S" Yet that is the only code I got.
I have two defouler plugs on her, but i still get this code, she does how ever go off every now and then and stays off for random increments of time, but always seems to return.
I was expecting it, but I was just wondering if I was getting terrible gas millage because of it? 220 miles to 14 gallons about, driving moderately, redlining daily.
I did search for this specific issue but found no resolve.
So should I just get the Cobb and delete it or, replace the O2, or just not pay attention to it?
I have two defouler plugs on her, but i still get this code, she does how ever go off every now and then and stays off for random increments of time, but always seems to return.
I was expecting it, but I was just wondering if I was getting terrible gas millage because of it? 220 miles to 14 gallons about, driving moderately, redlining daily.
I did search for this specific issue but found no resolve.
So should I just get the Cobb and delete it or, replace the O2, or just not pay attention to it?
Last edited by HexRX; 06-04-2012 at 01:42 AM. Reason: Typo ^-^
06-04-2012, 01:32 PM
#5
^ that is absolutely insane. would love to get that, but i'm in the same boat as Everest. Just got a warranty replaced engine too less than 1k miles ago, BHR coils.
i'm exclusively city driving though...
i'm exclusively city driving though...
06-05-2012, 02:33 AM
#8
Hexrx
Thread Starter
Does the cobb come preset with tunes or can I buy generic tunes from some one? o.o Quite curious, and I guess I will search ><
I got 230//13.68 = 16.81 this tank.. lil better
Also, 2004 MT 70K Miles on her ^-^
I got 230//13.68 = 16.81 this tank.. lil better
Also, 2004 MT 70K Miles on her ^-^
06-05-2012, 06:44 AM
#9
yes, search. others report defoulers don't work on 8s. 17 mpg in mixed driving with a 70k mile engine isn't unusual bad: http://www.fuelly.com/car/mazda/rx-8 The cobb comes with several tunes, but they need messing with for best advantage. Made sure your car has had the MSP-16 flash done - it ups the oil injection rates and helps the flooding issue. The dealer will be able to tell from his online records if it has. The update itself should be free.
06-05-2012, 06:48 AM
#10
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Your O2 sensors, including your rear sensor is actually used for fine tuning the cruising highway mileage, so having a working rear O2 can indeed help your mileage a bit. 14mpg isn't just an O2 failure though.
The reality is, a healthy RX-8 will get 17-18mpg city, 22+mpg highway. But 'healthy' means completely healthy: healthy coils, wires, plugs, O2 sensors, carbon free engine, good compression, healthy cat (or no cat), good transmission/diff fluid, no vacuum leaks, clean MAF, clean ESS. Let even one of these slide, and your mileage WILL drop, and it will drop fast. If you believe that RX-8s simply can not get this mileage because you have never seen it, then there is only 1 person to blame, and that is YOU. If you don't get this mileage, then there is something wrong with one of the above points. Or with your foot. Your right foot can make the mileage drop quite a bit as well. And I don't mean granny driving as the answer to good gas mileage. Smoothness is more important than speed. Smooth and fast is better mileage than unstable and slow.
So if you can't get this mileage, start poking around for what is wrong.
So if you can't get this mileage, start poking around for what is wrong.
06-05-2012, 12:38 PM
#13
Hexrx
Thread Starter
Not really any true issues, just wondering if the code would cause lower MPG, I did a search, Site:RX8club.com P0139 bad mpg, and other combinations.. I was just curious if it would cause such an issue, I did search and find a pleathura of information though on how to improve it if i was low.. But according to the forum, I am within normal range.
06-05-2012, 12:42 PM
#14
Hexrx
Thread Starter
06-06-2012, 02:33 PM
#15
I never run my car under 1/4 tank of gas. It's just bad for the car and makes the fuel pump work harder than it needs too.
06-07-2012, 08:25 PM
#16
I have never heard of that. As far as I know (unless the RX-8 downstream O2 sensor's system is totally different than other cars) the sole purpose of the rear O2 sensor is to monitor the catalytic converter's performance by comparing the two sensor (pre and post cat). I don't really see how the ECU could adjust fuel trim based on exhaust after the catalytic converter.
06-08-2012, 07:59 AM
#17
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I have never heard of that. As far as I know (unless the RX-8 downstream O2 sensor's system is totally different than other cars) the sole purpose of the rear O2 sensor is to monitor the catalytic converter's performance by comparing the two sensor (pre and post cat). I don't really see how the ECU could adjust fuel trim based on exhaust after the catalytic converter.
12-17-2012, 05:52 PM
#18
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I too have really bad MPG, but its not due to the O2 Sensor, its simply because you deleted/gutted yout CAT. I didnt believe it either when the dealer told me I need to get a new CAT, so I borrowed one from a friend and sure enough my MPG went back to normal and my CEL went away. I didnt think having a CAT would help with MPG but in these newer cars they do. Before gutting CAT after it went bad I was going 235 miles to a tank and now I am lucky to go 150 miles before the light comes on. But I also cant afford the $1400 the dealer quoted me for a new one. Also when mine went bad and blew my motor I didnt even get a CEL saying it was going bad. I am not about to spend another 6k on a new motor.
12-18-2012, 03:20 AM
#19
Hexrx
Thread Starter
Hmm, that sounds like a different issue, with my cat gone I get about 220 miles to the tank, I drive a bit aggressive though.
My issue, it turns out, was bad plugs, wires, and coils. Getting on average if I don't drive hard about 21 mpg.
My issue, it turns out, was bad plugs, wires, and coils. Getting on average if I don't drive hard about 21 mpg.
12-18-2012, 04:59 PM
#20
ruthless8
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well I wonder if I should replace my Air Fuel sensor, I have new plugs, wires and coils and no bueno. I have 187k miles on my car with original sensors.
12-18-2012, 06:13 PM
#22
I too have really bad MPG, but its not due to the O2 Sensor, its simply because you deleted/gutted yout CAT. I didnt believe it either when the dealer told me I need to get a new CAT, so I borrowed one from a friend and sure enough my MPG went back to normal and my CEL went away. I didnt think having a CAT would help with MPG but in these newer cars they do. Before gutting CAT after it went bad I was going 235 miles to a tank and now I am lucky to go 150 miles before the light comes on. But I also cant afford the $1400 the dealer quoted me for a new one. Also when mine went bad and blew my motor I didnt even get a CEL saying it was going bad. I am not about to spend another 6k on a new motor.
12-19-2012, 10:31 AM
#23
ruthless8
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I had a $100 Magnaflow #59959 metallic substrate cat (and a BHR resonator) welded into a $75 used RX7store midpipe. ~20k miles later, no CEL and a running average of 21 mpg. Though I have no experience with it, Magnaflow also makes a replacement cat/resonator for ~$300: 2004-2009 Mazda RX-8 Catalytic Converters - Magnaflow 23909 - Magnaflow Catalytic Converters
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