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My daughter has an '00 Accord with 215K miles. Recently she had the P1259 and P0135 codes. I replaced the front O2 sensor. I am working out the P1259 issue.

In addition, her mileage has tanked recently.

I replaced the pre-cat o2 sensor today. I also purchased a scan tool. The data from the scan tool looks good, however, one sensor looked out of whack. The scan tool was showing the IAT at 122 degrees. It was about 50 here today.

Should I replace/clean the IAT or leave it alone? Could the mileage issue be caused by the high readings on the IAT?
 

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I would replace it if cleaning doesn't solve the problem.
 

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Second gear slammer!
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clean the iat sensor. was the engine warm before you checked it? sometimes the manifold heatsoaks and will give an inaccurate reading. try cleaning your throttle body as well. the idle ports may have grime in them , and the butterfly may be dirty as well.

if anything i would think the iat readin hot like that would lean out the mixture rather than richen it up.. the o2 you replaced probably has or will fix your gas milage problems, if it wasnt the tbody all gunked up.

have you scanned your car hot and cold? if you have an ohmmeter you can ohm the sensor out and see if its any good or within parameters.
 

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clean the iat sensor. was the engine warm before you checked it? sometimes the manifold heatsoaks and will give an inaccurate reading. try cleaning your throttle body as well. the idle ports may have grime in them , and the butterfly may be dirty as well.

if anything i would think the iat readin hot like that would lean out the mixture rather than richen it up.. the o2 you replaced probably has or will fix your gas milage problems, if it wasnt the tbody all gunked up.

have you scanned your car hot and cold? if you have an ohmmeter you can ohm the sensor out and see if its any good or within parameters.
A dirty throttle body should have zero impact on fuel economy if all of the sensors are working correctly. The computer measures the amount of air, and adjusts the AFR accordingly (assuming the sensors aren't clogged in the process).

Checking the computer's IAT reading is going to be a much greater troubleshooting tool than an Ohmmeter. Unless you have the sensor's resistance curve and a very accurate meter, your values are going to be just a tad off. Checking the computer will tell you exactly what the computer is reading, taking into account the full resistance of the system, etc.
 

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Compare IAT and ECT with a cold engine- they should be the same. 122 sounds normal with a warmed engine. The IAT would have very little affect on fuel economy. Can your scanner check fuel trim now that you've changed the O2 sens? Do you know what it was before?
 

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The IAT reads that high with a warm engine because Honda has chosen to put the sensor in the intake, which heat soaks vs in the airbox or intake tube to the throttle body.

The gen 7 v6 engines have a big problem with IAT heat soak in hot weather. After a hot soak (engine off) and restart the car (mine is anyway) a dog until outside air is able to cool the intake charge.

The P1259 could be a cause of the lost mileage along with the air/fuel sensor heater failure.

Is there oil in the car? First. Is, or has the car been bucking severely as you try and rev it beyond about 3,500 rpm?

If there is oil, you'll need to verify that there's enough oil pressure to activate the vtec system. You can take off the vtec solenoid and check the gasket screen, if it's got junk in it, you'll need to find out where it's coming from.
 
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