1992 N14 1500 max

This is a 92 IHC with an N14. The only diagnostic connection I can find is on the left kick panel. It is small, the size of my thumb. I have nothing to connect to it.
The speedometer sensor is a single with four wires. The speedometer works and swapping the connectors made no difference.
Holding the DIAG toggle ( KOEO ) Will flash 21, 22, and 23 at times. Possibly I miscount at times but I find no information on these numbers.
After researching I will replace the fuel filter. It is skipping ( Miss firing ) on number 4. Looks like wiring harness can cause this also?
Any experience on this appreciated.