Just like one of the techs I work with says: if you don't schedule your maintenance, your maintenance schedules itself.
During our routine 180-day compressor service, I noticed an overheating error in the logs from the day before. That couldn't be right. How could we have been so lucky? We were due for a full service anyway (oil change, oil air separator, filters, etc.) so we figured we'd proceed and evaluate the fully serviced machine.
After the service, we felt the air coming from the cooler. It should have lined up with the temp reading, but it was cool air, about 120°F, not even close to the 212°F the oil temp was reading. Since the cooler should have the oil passing through it to cool it the air should be at least somewhat close to the oil temp. Clearly the oil wasn't making it to the cooler.
We pulled the thermostat. It should open when the oil gets hot, allowing it to the cooler instead of directly back to the sump. We used a heat gun to simulate heat, but it wouldn't open. We like to call that a "smoking gun." We had a spare on the service van and swapped it out. Oil temp was spot on after the fix.
What perfect timing for our service. We seriously couldn't have timed that repair any better, coming the day after the oil temp alarmed.
...and yes, there are ways to get these wired into the internet to alert us proactively, but that's another conversation for another time..