The standby instruments in the Mustang are fairly simple. They are used most frequently in flight as a bug for the baro setting when descending from the flight levels to the transition altitude. They are not approved for RVSM so if you lose an AHRS in RVSM airspace, you must declare to ATC that you are no longer RVSM compliant. ATC may ask you to descend. Additionally, the standby instruments get all their data from the left hand pitot static system, so if you lose your left pitot static system, you've lost standby instruments as well.

In checklist, they are referenced only once:

TAXI

3. Flight Instruments (including standby instruments) ... CHECK

They can be checked to see if they are in agreement with two PFDs. However, there are also two green LEDs that must be illuminated to ensure that the instruments are working correctly. If the two green LEDs are not lit, then the plane is grounded.

There is diagnostic software now available that can reset the diagnostic lights. In most cases, a simple reset is required. The diagnostic instruments have a small USB connector to run the diagnostic software. See Cessna Support for details including the software download.