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My son's 2010 Civic w/1.8L engine is getting a P0128

I used the Topdon Topscan to check the various values and we will replace the thermostat

I noticed the IAT value seemed way off at like 109° F sitting in the driveway engine off while the ambient temp was like 55° F.

Drove around a bit at highway speeds and it dropped a bit to 104° - 99° F with similar ambient temperatures

This seems way off

Does anyone have any data from another engine?

Matt

 
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Was that 109F after it had been running? Or was that after sitting overnight? If it's sunny and parked on pavement, under-hood temps could be quite a bit higher.

 
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it's after a 35 min / 20 mile drive on the freeway (70-75 mph)

It was a cloudy day in the shade after sitting for about 10 minutes, as I stated we took it out and drove around at freeway speeds and the value only changed about 5° F from 109 to 104

Seems to me this would affect the emission control system calculations

Can you plug in a scan tool and see what values you get?

The research I've done so far indicate the value should be roughly 5° to 15° different

 
Rep Power: 223

I don't own an 8th gen Civic, can't help you there. I can see what my CRV says if I remember later.

Let it sit overnight so you know it's completely cooled down. Then read IAT in the morning before starting the car. Should be very close to ambient temperature. 104°F IAT does seem a bit high for 55°F ambient temperature.