Welcome, I am Chris (aka-Moose)
The diag fee at a dealer to figure out what is wrong through a scan tool and testing, is about 95$.
The repair cost parts/labor can range from as low as 150$ for a height sensor to as much as about 900$ for a module replacement.
On the towing question. If the ride height is correct and the vehicle is not sagging or swaying then yes you could tow with this warning on, its not optimum or approved by GM but you could.
In case you did not know, many air ride car owners often opt for a spring suspension conversion, which often comes out cheaper to install compared to repair and there are no more issues after that.
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