The simple answer is that your Freeze Frame Data is a 'saved' profile of all
your emissions/operating parameters (usually the MAP/MAF have greatest fuel
authority then the EGR/O2 sensors) at the time the CEL code is generated.
In otherwords, fuel trim percentages help troubleshoot drivability issues
based on comparing sensor outputs from most of the 5V reference emissions
sensors on your vehicle.  If your short term fuel percentage shows +28, this
means that your computer is adding 28% fuel to compensate for a lean
condition (could be a bad O2 sensor, fuel leak, bad plug, EGR....).  Looking
at both fuel trims levels can help troubleshoot IF- THEN situations a little
more clearly.

Fuel Trim ratings simply allow us to measure whether fuel is being added or
taken away from baseline fuel duty cycles.  Short term is basically
adjustments on-the-fly to fuel delivery and long term is factored as a
result of 'x' amount of drivability cycles to give you a 'long term' average
that might eliminate minor adjustments, etc..  As we all know, this is a
half-a.ses explanation as there are many variables and speciific timing
involved in the correct 'technical' answer so you would best be served by
looking at something like this:

http://www.asashop.org/autoinc/november/scantool.htm

(MUCH better explanation).

Take care,
Andrew.

in article 1182357026.042783.286480@q75g2000hsh.googlegroups.com,
jeff.severson@hotmail.com at jeff.severson@hotmail.com wrote on 6/20/07
12:30 PM:

Unless you have a specific problem, those scanners are totally useless
in my opinion.

The data they display can be compared to the engine's specs, but there
is dick all you can do about any of them.  There is never a hope of
getting then the same all the time due to weather, temp, physical makeup
of fuel, and on and on.

They sure must be a money maker for someone though with people tossing
parts at scanner 'ghosts'....

Just my $0.02

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's - Gone to the rust pile...
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