Epson 20 - Manual (Page 143)

If another failure is found, you see one of these messages:
or system unit error
or system unit error
Display Card Check
The computer checks the color or monochrome display adapter card
that is installed in the computer. An error number and message
appear if any faults are found. Number 401 represents an error in the
monochrome display adapter, and 501 represents an error in the color
display adapter. The messages are:
401-CRT error
501-CRT error
Optional RAM Check
If any additional memory (expansion RAM) is installed, the computer
then checks that RAM for any malfunction.
Floppy Disk Drive Seek Check
The computer checks its floppy disk drives by searching the read/
write heads for any malfunction.
If any seek errors are found, you see this message:
601-Diskette error
If this error occurs, confirm that the system diskette is inserted into
drive A. If this number still appears after you insert the diskette, the
disk drive must be repaired.
B-4 Power-on
Diagnostics