Help with fuel trims.

[Pvt-Public]

1999 Camry 2.2, AT Calif Emmisions.
Could someone please explain in layman's terms the meaning of long and short term fuel trims? It is in percentage form but percent of what? I get an occasional P0420 code for catalyst efficiency below threshold. Here is the freeze frame data any ideas?
DTC that caused freeze frame, 0420
Engine RPM 1763
MAP Sensor 66 KPA
Coolant Temp. 186F
Intake Air Temp. 75F
Short term Fuel Trim 1 3.1%
Long Term Fuel Trim 1 -100.6%
Long Term Fuel Trim 3 -100.6%
Vehicle Speed 33 mph
Fuel System 1 CLSD
Fuel System 2 n/a
Thanks for any help!

[fenixus]

fuel trims are the percentage of commanded (by ECM) change in fuel delivery amount over the ideal stoichiometric A/F ratio (14.7:1).
it can be either negative (less fuel delivered for same oxygen amount) or positive (more fuel delivered for same oxygen amount).

you can google it a bit, i'm sure there will be dozens of interesting articles on fuel trims, o2 sensor readings, MAP readings, EGR involvement and other things that can play a role (like vacuum leaks).

http://www.motor.com/magazine/pdfs/102003_05.pdf

http://www.motor.com/magazine/pdfs/122005_05.pdf

these are 2 I googled quickly, there is much more on that topic.

Your P0420 may indicate a problem with catalytic converter efficiency below treshold. but you have to remember that rear oxygen sensor is used for determining so by the ECM. so my advise would be to replaced that rear o2 sensor with Denso aftermarket part first and go from there.