Curious LED failure mode

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On my electric gate/ intercom, there is red LED to show the gate status.
Lately this LED fails - 3 times so far in a year. But each time, it doesn't die completely, it suddenly goes really dim - yet still works.
It's switched via a dedicated relay on the microprocessor board, powered from 12V dc, and the series resistor is 1k5, so not excessive current, and no obvious reverse bias.

When I replace the LED, the new one work fine instantly, nothing else need repairing.

Is this a known failure mode for LEDs? I've never known them to suddenly go dim, yet still work. The forward voltage drop after failure is rather high at over 2V.
 
I've seen similar behaviour several times from LEDs that have been driven with too much current.

If you have just a standard 5mm LED in there (or something similar) I'd be willing to bet the 6.7mA you have flowing through it is too much and over time it is cooking its self (especialy if the housing it's in can get hot). So you could try swapping out the curent limit resistor for somethinig in the region of 3k or more and see if this stops your problem.
 
Maybe they are actually low current LEDs. Try to run one at 5 mA and see how bright it is...

I've seen something similar after a burn in period on some of our products. In one batch a few LEDs had not survived the burn in period (at 70 degrees C) and then the baking of the conformal coating at 90 degrees C.
 
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