Importance Of Maintaining The Def Level - Ford Diesel 2011 Supplement Manual

Maintenance and Specifications
WARNING: The fuel system may be under pressure. If the fuel
filler cap is venting vapor or if you hear a hissing sound, wait
until it stops before completely removing the fuel filler cap. Otherwise,
fuel may spray out and injure you or others.
WARNING: If you do not use the proper fuel filler cap,
excessive pressure or vacuum in the fuel tank may damage the
fuel system or cause the fuel cap to disengage in a collision, which may
result in possible personal injury.
Selective Catalytic Reduction (SCR) System
Your vehicle is equipped with a selective catalytic reduction (SCR)
system to help reduce emission levels of oxides of nitrogen from the
exhaust of the diesel engine. The system automatically injects diesel
exhaust fluid (DEF) into the exhaust system to enable proper SCR
function.

Importance of maintaining the DEF level

In order for the SCR system to operate properly, the DEF level must be
maintained. Generally, the DEF tank should be filled during the oil
change service interval. See the scheduled maintenance information in
this supplement for more information. However, certain conditions or
driving styles, such as trailer towing or fast rates of acceleration, will
require the refilling of the DEF tank more often.
The engine control unit will monitor the amount of fluid available in the
DEF tank. Running a system check in the message center will indicate
whether the DEF level is ok or if it is less than 1/2 full. A message will
automatically be displayed in the message center when the DEF level is
low and needs to be refilled. When you see this message you should refill
your tank. See Message center in the Instrument Cluster chapter of
your Owner's Guide for message center functions. For instructions on
refilling your DEF tank, see Filling the DEF tank later in this section.
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2011 Diesel (eco)
Supplement, 5th Printing
USA (fus)