Recently I found my AT temp sensor read ~10F lower than my ChinaFreight temp gun. "The hell..." I thought. "I'z a engineer... need data."
So I got out my new(to me) Fluke 87 Digital Volt Meter with Included Thermocouple Sensor. Those cost ~$400 new (got mine used!) and are reputed to be very accurate.
The Fluke reading was within 1F of the AT sensor.