Which Chronotherm III model? The ones I've seen have the system light to the right of the word SYSTEM, not under it. Anyhow, there might have been variations, does not sound like a big deal of a difference.
So, yellow light next to SYSTEM just means that the thermostat is calling for either heating or cooling, depending on which mode it's in. Given that it's summer (here in the North Hemisphere), I imagine it's on COOL, so the AC should be ON. Does the AC run?
I just double checked and the yellow light is under the word "System" I live in mid century modern house built in 1959 so this is a older unit, I'm sure not a 1959 model but older for sure. The A/C is not running which here in Dallas is a problem at the moment. I have checked the power sources and everything seems normal. I was curious if this yellow light is telling me something. I turned the system clear off and back on again. The yellow light did not come at first but within a couple of minutes it appeared again???
PS I don't see a model number it just says Honeywell Chronotherm III
It looks like the thermostat is actually doing its job - calling for cooling when it's warmer than its COOL setting. So, unless you also have REPL BAT flashing on the display, I would say the problem with AC lies elsewhere in the system.
What do you have as the AC outside unit and what is the air handler inside? When things stop working, it's high time to start writing down model numbers / manufacturer names of all the equipment involved - this info will be really helpful.
Is any part of this system working? For example, air is moving but isn't cool. That would mean that the air handler is working but the outside unit is not. Have you checked if the circuit breaker on the circuit powering the AC is tripped?
P.S. Chronotherm III is an old model but not as old as the house, I don't think. They have been around in the 80s; earlier than that - I don't really know. Regardless, your issue appears to be elsewhere.
I just moved in so I am researching and learning.  I have checked the power sources and all seems ok. There is no air moving. I diving in deeper.
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I have your thermostat also. Upper right corner is green light under word Enrg Sav. Next to it is yellow light under word System. Both lights glow all the time. Is this normal?
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