To perform a complete test of the hp 12c electronic circuity:

• Turn off the calc.
• Press and hold [x] (multiply) button and at the same time press the [ON] button.
• Release the [ON] button.
• And then release the [x] button.
• You should see the "half Running" message flashing on the display
• After about 10 secs, it should display -8,8,8,8,8,8,8,8
  
• Off and On the calc, it should display 0.00

If you see "Error9" on the display, then the self-test was unsuccessful. You may want to go on to the following section, to perform a system reset.

If you like do run this test infinitely.......

• Press and hold [+] (multiply) button and at the same time press the [ON] button.
• Release the [ON] button.
• And then release the [+] button.

To stop the test, press any key and waif for a couple of seconds, it should display -8,8,8,8,8,8,8,8.

Happy Testing.