1990 Honda Civic Wagon 1.5
The Tachometer jumps or works fine or doesn't work.
There is no code showing.
What could this indicate?

The tach is bad, a connection is bad, or maybe the sending unit is bad.

It's possible the Ignition Control Module (ICM) is failing. The BLU wire from the ICM goes to the tachometer. An erratic tachometer is often a sign that the ICM is bad.