I'm getting these codes:
21 .............................. Solar Sensor Circuit
42 ..................... Air Inlet Door Servomotor Circuit

And a code 24. What's the code 24? (source: AC codes)
Code 42 - yep, lots of noise.
Not concerned with the 21 at present, AC&heat are operating. Should I be?
Thanks!

You can ignore code 21 most of the time. Once you get your servo fixed (code 42), you can watch for the fan speed to change when you're in direct sunlight vs. when you're in shade.

Chip H.

Thanks for writing. It's odd that there's no documentation for code 21 (that I have access to).

Code 21 is the solar sensor on top of the dash. If it's covered up or you're inside a building when running diagnostics, then 21 will pop up as it's not detecting solar radiation. In other words, nothing to worry about.

I'm chasing AC issues as well. I'm also getting 42, the fresh/recirc servo chatters some on first start up.

But my biggest issue is the AC goes from cold to warm, then I can drop down 2-3 degrees and it gets cold again. When it's cold, it's really cold! I'm 99% sure it's the air mix servo even though there is no code for it.

Recently, we were chasing a very strange AC issue where the compressor would kick on an off rhythmically, like clockwork. We went thru every single AC circuit and it ended up being the evap temp sensor. So lot's of time, effort and a long time without fully functional AC and it was a $20 part!

Update: after running the DX and cycling through all the testing, no more noise in normal operation. Everything appears to work. Gotta love aging equipment.

Sounds like my house HVAC. Not really. I just miss good bimetal mercury thermostats. Heat pumps are a truly strange way to heat/cool. Right when it's hottest/coldest you're trying to extract/exhaust energy. A little integration with weather forecasts would go a long way. And don't even think about anything more than 2-year warranties. Digital thermometers are subject to the same issues as the rest of the computer industry + the problems with digital/analogue interfaces.