[John S Peterson]

If you had an air fuel sensor (wide band) you could use Lambda calculations to really tell what was going on with AFR, but unfortunately very few vehicles at this time other than european vehicles use them. OBD II uses an O2 sensor monitor that checks response time as well as checking the heater circuit which will trigger the MIL. Any fuel trim numbers out of the norm for any length of time will usually turn the MIL on and if emissions exceed 1.5 times the Federal Test Procedure the light will flash indicating extreem danger to the CAT. If anyone is interested I have a lambda calculator that you can just plug your 4 or 5 gas readings into to get what your REAL AFR is, this is great for tuning you just have to have your gas readings to use it.