HP 48G Series Quick Start Manual page 112

If you suspect that your calculator is malfunctioning:
1. Run the self-test:
a. Turn on the calculator.
b. Press and hold (ON).
c. Press and release the "E" menukey.
d. Release (ON). The diagnostic self-test tests the internal ROM
and RAM, and generates various patterns in the display. The
test repeats continuously until you perform a system halt.
2. Halt the self-test (system halt):
a. Press and hold (ON).
b. Press and release the "C" key (the key with C next to it).
c. Release (ON). The empty stack display should appear.
If the self-test indicates an internal ROM or RAM failure (if IFO Qi
and IFAM OF are not displayed), the calculator requires service.
If the calculator passes the self-test, you may have made a mistake
operating the calculator. Reread appropriate sections of the
documentation and "Answers to Common Questions" in Appendix A
of the HP 48G Series User's Guide.
9-4 If Things Go Wrong ...