[John S Peterson]

Fuel trim in numbers is the addition or subtraction of fuel as required to maintain as close to 14.7:1 as possible. You have both short term and long term corrections that will allow the cat to work as efficiently as possible. OBD II's only job in a nut shell is to protect the CAT so in a sense every OBD II vehicle has it's own 5 gas analyzer on board. Pretty cool if you think about it.