I believe on most models, the light is set to come on when there is 3mm of pad left on the front, and min thickness is 2 mm. Some models have rear sensors too.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record