Looking at Trouble code chart for your vehicle does not display trouble code 18. It shows trouble code 17. Next trouble after 17 is 25. Please have a qualified mechanic perform a diagnostic of your Onboard Computer System and give you proper trouble code.
To Answerer No. 1: Yes, my vehicle DOES display trouble code 18: I checked with Alldata and Mitchell Automotive software (other person has this software). This is what the table of trouble codes states:
The table lists various codes, including the codes 17 and 25,
then it clearly states, in BOLD CAPITAL letters:
"CODES NOT LISTED - PGC, See: Powertrain Management/ Computers and Control Systems/Testing and Inspection.../Pinpoint tests/PGC - Power and Ground Connections"
So, it directs me to go through tests of Computer's power and ground connectors, and possibly inputs on computer from various sensors. I could go through all that, but I figured that experienced mechanic (not a dude from the street like the one who said "there is no code 18") would narrow down the tests for me based on his/her experience with these symptoms.
Moreover, various online sites (just Google "ford trouble codes") give description of code 18 and three reasons for it.