Furuno FELCOM501 Operator's Manual page 171

When pressing the Enter key on the "RGB gradation" screen, the screen changes
as shown below.
7. Press or to select [OK] or [NG], then press the Enter key. The [Key Test]
screen appears.
8. Press the Enter key to start the key test.
9. Press each key. The related on-screen location fills in blue if the key operating
normally. If the key is pressed again it changes color in the following pattern: white
→ blue → white...
10. To go to the hook test, press the Enter key three times.
If all keys are not tested at this time, the test result for the keys is shown as "NG".
7. MAINTENANCE AND TROUBLESHOOTING
7-7