The gas emission light coming on because the gas cap is worn out or the gas cap is not fitted properly.Its got loose.remove, clean the gas cap and reconnect.If still the same problem, then replace the gas capo.If still the same problem, then its advisable to get the car scanned.at autozone service center its absolutely free to get car scanned.They don't charge for scanning, only charge for parts replaced.--------as you mentioned that gas emission light comes on, this indicate there is error stored in your cars computer memory/.In scanning that error code will be retrieved, as per error code it will directly point towards the faulty part or sensor causing the gas emission light to come ON.-----------
Most probably in scanning you will get any of the two possible codes:----
The code P0455 is for a gross leak from the EVAP (fuel vapor) system.
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P0446 is for cannister vent valve circuit failure.-------
For more details, click the link below:---
http://technoanswers.blogspot.com/2011/06/ford-explorer-eddie-bauer-has-gas.html
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This should help.Thanks.Helpmech.