Rep Power: 0

I've always gotten between 35-39MPG in mt 2015 Civic. About a month ago, my display said it was time to change my spark plugs. About 2 weeks ago, we had a crazy cold snap when it was below zero for several days. During the cold weather, the check engine light came on and it said something about an evap system malfunction. when the weather warmed up a little--still below freezing--the check enging light went off. Driving in the snow, having the car idling in parking lots, etc, I didn't pay attention to my gas mileage at the time. I went to autozone and checked the codes. it said I had an evap purge/flow issue. I bought some NGK Iridium plugs and changed them that day. Ever since, I'm getting about 28MPG. The plugs were gapped right. I swithced them out with the old plugs, and the milesge is still around 28MPG. Any idea what is causing the drop in gas mileage?

 
Rep Power: 367

what mileage?
typically, o2 sensors on their way out would cause MPG drops
moving thread to proper location

 
Rep Power: 222

Still have the evap code? That could be making it run extra rich compared to normal when you start it and it's trying to get up to temp

 
Rep Power: 0

PO497 EVAP Canister System Low Purge Flow
P145C EVAP Syestem Purge Flow Malfunction
PO496 EVAP System High Purge Flow Detected

The light came on when it was 15 below zero. The light has stayed off ever since. I figured maybe the vavle got stuck in the cold temperatures. When it got up to about 20 degrees, the light went off and is still off.

If the light is off, could it still be malfunctioning?

 
Rep Power: 0

PO497 EVAP Canister System Low Purge Flow
P145C EVAP Syestem Purge Flow Malfunction
PO496 EVAP System High Purge Flow Detected

The light came on when it was 15 below zero. The light has stayed off ever since. I figured maybe the vavle got stuck in the cold temperatures. When it got up to about 20 degrees, the light went off and is still off.

If the light is off, could it still be malfunctioning?

 
Rep Power: 0

It has about 105,000 miles. But the drop in gas mileage was sudden. It was getting 35–39MPG, then immediately after the cold snap (when I only drove a total of about 20 miles) it started getting about 27–20MPG.

 
Rep Power: 0

The original plugs were gapped at about .005 when I removed them. The new NGK Iridiums I put in came out of the box at .042, but were supposed to be pregapped to .044. I adjusted them to .044 and my gas mileage immediately dropped to 27–28MPG. I regapped the origianl plugs to .044 and reinstalled them, and the MPG is still 27–28. I wonder if I should try opening the original plugs back to .005 and see it the mileage improives???? It's supposed to be .044.

 
Rep Power: 367

reset codes with OBD reader if you have one.

if not,
get your radio code (not sure if 8th gen have them or not, but...)
disconnect battery for a while
this will reset computer
go from there.