Lyle, as mentioned it is the brake pad wear indicator. As far as the urgency to replace the worn brake pads, it depends on what type and how many miles you drive in a week/month. I typically try to replace worn pads within a month after the indicator illuminates, and there is still plenty of pad thickness remaining. Pull the wheels and check the pad thickness. Obviously you want to avoid wearing through the brake pads and have the backing plates score the rotors. Remember to buy/install new wear indicators with the new brake pads.

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Fred Hoelzle