MPG gone South
12-30-2013, 08:31 PM
#1
Thread Starter
MPG gone South
We had a check engine light come on and it said it was the forward O2 sensor(it came out black and had a light coating of soot on it). Replaced it with a new Denso 2349065 and now my mpg is down to 21 mpg but hey I don't have the check engine light on any more. 
We use to get 28 to 30 mpg on our '06, to drop 8 or 9 mpg is not right but the car runs fine all the way to red line. I track all my mileage in the S so I know when something is going south but this has me stumped. How could the mpg drop that much and NOT turn on the check engine light or throw a code to read?
ROD
We use to get 28 to 30 mpg on our '06, to drop 8 or 9 mpg is not right but the car runs fine all the way to red line. I track all my mileage in the S so I know when something is going south but this has me stumped. How could the mpg drop that much and NOT turn on the check engine light or throw a code to read? ROD
12-31-2013, 12:59 AM
#3
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AFAIK:
The only CEL that will tell you an O2 is broken is the "heater circuit short" one.
And then you will find it is actually broken, that usually only happens with the secO2 being decapitated by the first cat core sliding back.
An O2 being black/coverd in soot means little, the whole exhaust is black on the inside.
There is nothing to hit the primO2 to damage it, so what CEL did you have to make you swap primO2's?
If the original primO2 was out of range there was/is something else going on.
Did you reset the ECU after installing the new O2?
The only CEL that will tell you an O2 is broken is the "heater circuit short" one.
And then you will find it is actually broken, that usually only happens with the secO2 being decapitated by the first cat core sliding back.
An O2 being black/coverd in soot means little, the whole exhaust is black on the inside.
There is nothing to hit the primO2 to damage it, so what CEL did you have to make you swap primO2's?
If the original primO2 was out of range there was/is something else going on.
Did you reset the ECU after installing the new O2?
12-31-2013, 07:15 AM
#4
Thread Starter
Why NGK? The one that came out was a Denso and it was the original '06 o2 sensor.
The two codes I got were, P2A00 and P1172 and that is for the forward o2 sensor. I took the back one out to see if it had any damage to it and I found none. I could stick my finger in the o2 hole and feel the cat and I could not feel any lose or broken pieces nor could I move it with my finger. If the cat was bad(still have the original rear o2 in the car) I would still get a code and I've put 300+ miles on the car with no codes so far. I did not reset the ECU but I did clear the codes.
ROD
AFAIK:The only CEL that will tell you an O2 is broken is the "heater circuit short" one.And then you will find it is actually broken, that usually only happens with the secO2 being decapitated by the first cat core sliding back.An O2 being black/coverd in soot means little, the whole exhaust is black on the inside.There is nothing to hit the primO2 to damage it, so what CEL did you have to make you swap primO2's?If the original primO2 was out of range there was/is something else going on.Did you reset the ECU after installing the new O2?
ROD
12-31-2013, 08:30 AM
#6
if its the primary it will be covered by the TSB.
i think this might apply to you.
https://www.s2ki.com/s2000/topic/782...c-p1172-p2a00/
i think this might apply to you.
https://www.s2ki.com/s2000/topic/782...c-p1172-p2a00/
01-01-2014, 06:09 AM
#7
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OP: I'm sure you've Googled P1172.
P1172 Honda - Rear A/F Sensor Bank 1 Sensor 1 Circuit Out Of Range High
Don't get "foolded" by the Rear A/F, it is still the prim O2 because of the Bank1 Sensor 1, the sec O2 is Bank 1, Sensor 2
P2A00 Honda - Air/Fuel Ratio Sensor Sensor 1 Range/Performance Problem
As far as I know, the sensor reads high in output when the mixture is/was too rich.
edit: well.... the TSB is pretty clear.
I say: case closed.
P1172 Honda - Rear A/F Sensor Bank 1 Sensor 1 Circuit Out Of Range High
Don't get "foolded" by the Rear A/F, it is still the prim O2 because of the Bank1 Sensor 1, the sec O2 is Bank 1, Sensor 2
P2A00 Honda - Air/Fuel Ratio Sensor Sensor 1 Range/Performance Problem
As far as I know, the sensor reads high in output when the mixture is/was too rich.
edit: well.... the TSB is pretty clear.
I say: case closed.
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01-21-2014, 04:49 PM
#8
Thread Starter
Replaced the rear o2 and my mileage is back up to where it was before the rear o2 went bad. 28+ mpg in the winter(even if its not cold at mid day) starts out around 32F and goes up to 67-71F by mid-day. Good to be able to drive the S again. 28 beats the hell out of 15 mpg.
ROD
ROD
01-21-2014, 07:02 PM
#9
so you replaced the rear anyways even though it wasnt throwing a code for it? ill be replacing mine soon. i probably should replace the primary as well. especially i still have yet to get the updated flash from honda. i think i will just buy the flash pro and do it myself since i know the updated flash will be on the flash pro. i dont want to take a chance and install the new sensor and have them flooded out. 
thank you for coming back and sharing your solution.
thank you for coming back and sharing your solution.
