Ford Vehicle Owner's Manual page 72

NOTE: If your light truck is equipped with
dual fuel tanks, the "CHECK ENGINE"
light may come on if fuel is restricted
to the engine or if the fuel flow is
momentarily disrupted because of an
empty fuel tank before you switched to
the auxiliary fuel tank. This condition
is normal and the light should go off
after full fuel flow is restored.
An example of a condition which corrects itself
occurs when an engine running out of fuel
begins to misfire. In this case, the Check Engine
Warning Light may turn on and will then set a
Diagnostic Trouble Code indicating that the
engine was misfiring while the last of the fuel
was being consumed. After refueling, the Check
Engine Warning Light will turn off after the
vehicle has completed three consecutive warm
up cycles without a misfire condition occurring.
A warm up cycle consists of engine start from a
cold condition (engine at ambient temperature)
and running until the engine reaches normal
operating temperature.
On the fourth engine start up, the Check Engine
Warning Light will turn off as soon as the
engine begins to crank. It is not necessary to
have the engine serviced.
Under certain conditions, the Check Engine
Warning Light may come on if the fuel cap is
not properly installed. If the Check Engine
Warning Light comes on and you suspect that
the fuel cap is not properly installed, pull off the
road as soon as it is safely possible and turn off
the engine. Remove and replace the fuel cap,
making sure it is properly seated.
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