If the thermostat change was recent recent, you may have had a little trapped air in the system which finally burped. I'd just wait and see if it comes on again assuming your temp reading is normal. If ithe light comes on again, I'd either pressure check or replace the rad cap. This cap doesn't just close off the hole anymore like the old days. It has to be able stay closed under pressures up to 140 kilopascals. If it doesn't hold, you will loose fluid. The next step would be to do a system pressure check for leaks. First of all don't pressurize over system pressure or you might be buying a new heater core. Secondly you can't pressure check a hot system. The system must be cold or there is a chance you won't find the leak. Good luck,
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