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How Does It Know?

This morning, my 987 Boxster illuminated a warning message, “Check front left side light.” I checked the light, and everything seems fine, but I have reason to believe that the front left side light will soon need replacing.

Porsche 987 Boxster warning light.
Porsche 987 Boxster warning light.
Image: Dakotahound

Over the course of the past two years, I have replaced both dipped beams (low beams) and one side marker (actually, I replaced both side markers, but only one was out). About three weeks prior to each one failing, my Porsche gave me a warning. How does it know? I have some ideas:

1) The resistance of the bulb changes slightly before it burns out.

I don’t think that this is the case. The resistance of the bulb would normally change due to weather (hot and cold). Also, consider the case when the light has been on for a while and then the car is restarted. The bulb would be hot (with high resistance) during the start-up test.


2) The bulb becomes intermittent just before it fails.

This seems more likely. In fact, during the three weeks prior to the light failing, the warning is not constant. I get it about every other day, and it goes out after I drive a few miles.

3) This is a totally random event.

Not likely.

(This picture also made me realize that I need to clean my instrument panel.)

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