This question has popped up recently elsewhere. For years I was told not to mess with them, then now I am told you can? I never have and always go to tune to within what's there like on a 1227747 BLM min 108 and max 172.

If you change them, they are in tables, would it mess up the table and add to hex moving stuff to other adress making tthe mask wrong? Or can it be done?

I still don't know the purpose if any?

 

Yes they can be changed. They are a calibration entry.

The main reason to leave them be is they limit the amount of adjustment to the fueling that can be made. A silicone poisoned O2 sensor will drive the BLM down. If the lower BLM limit is low enough the engine will cut out and stall.

Not fun on the highway at 65 MPH. At that point may need to key-off for at least 5 seconds to reset the BLM values, then crank to start again. Note that depending upon the mask ID the BLMs may or may not be reset back to 128.

A cold O2 sensor usually reports lean. So the opposite can happen and foul out the spark plugs.

RBob.