I just replaced both belts because it had been 3 years old and had 90K miles since the last replacement. I routinely inspect the belts and had only noticed minor cracking and nothing that concerned me. However after removal and inspection, the larger belt had a relatively large split forming which I suspect would have led to belt failure in the near future. I'm posting a photo of the split. So keep an eye on the belts if they are getting up there in mileage or age.
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