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2004 Taurus SES Duratec
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857 Posts
What specifically triggers the check fuel cap light?
The system measures fuel vapor pressure through a couple of sensors. If it has set a check fuel cap light and no other codes, then the readings it got were consistent with a lack of seal at the fuel cap.
 

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2004 Taurus SES Duratec
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857 Posts
I do have a small evap leak but not a large. Code 657 I think at time of the light. Yes 456 and 457
First step is to check the fuel cap, in particular the gasket and where it seals for dirt and damage.

The computer will continue to monitor pressures and, if applicable after a few monitoring cycles, it will remove any code that it believes is fixed. Or you could clear the codes and see if they come back.
 

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2004 Ford Taurus SEL - DOHC
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160 Posts
Discussion Starter · #6 ·
So I pulled over, cut the car off, removed the gas cap and put it back on. Cranked car and it still said it, so I repeated the actions. After that it was still on, but it did go off within a mile or two of driving. The code no longer showed pending either, but remained a Stored code. The gas cap didn't seam loose when I checked it, but I suppose it could have been. I have recently noticed a drop in my gas mileage. Is there a chance the two could be connected?
 

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2004 Taurus SES Duratec
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1998 Taurus SE Sedan 3.0L24V AX4N 91Kmi
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152 Posts
What specifically triggers the check fuel cap light?
This page explains the earlier version of the system on a CV, but it's very similar on your car.

(click this text)


If tank pressure rises faster than it should at that ambient temperature, the system assumes it's from air leaking in, and it assumes you didn't get the cap back on correctly. It only shows the light while it detects the problem, but the code remains in memory like any other stored code: ~50 drive cycles after the last time that fault was detected, or until codes are cleared or KAPWR loses voltage.
I have recently noticed a drop in my gas mileage. Is there a chance the two could be connected?
It's not likely.
 

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2000 Taurus SES 12v
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992 Posts
It's not likely.
Today we read from the Book of Troubleshooting, Verse 24: Excessive fuel consumption:
  1. Dirty or clogged air filter element.
  2. Incorrect idle speed.
  3. Low tire pressure or incorrect tire size.
  4. Inspect for binding brakes.
  5. Fuel leakage. Check all connections, lines and components in the fuel system.
  6. Dirty or clogged fuel injectors.
  7. Fuel injection or engine control system malfunction.
  8. Thermostat stuck open or not installed.
  9. Improperly operating transmission.
Although it is Sunday, let us not go astray and assume the answer is 7. May the Wrench be with You.
 
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