This is typical of the electromagnet getting residual magnetism in its core. The amount of magnetism is such that once the moving arm touches the magnet core it will stick, but as soon as there is a slight gap the moving armature will fall away.

Try putting a piece of masking tape over the magnet pole, or alternatively on the moving armature. This will introduce enough gap in the magnetic path that they will not stay stuck.

I don't know which locomotive you're talking about, and I'm not familiar with any of the H0 locos anyway.

But; if you're comfortable with opening your loco and do a bit of soldering; I would try to swap the two wires going from the decoder to the telex coupler.

This will change the direction of the induced magnetism, and will help you for some time, after which you have to swap back.

Just an idea.

Per.

Good afternoon

The Märklin Telex coupler of recent generation are not perfect but most work reasonnably well.
I spent hours on trying to find the problems. Here are my findings:
- never oïl, clean, etc the mechanism.
- some of the coupler holder are not precise enough thus resulting in car uncoupling even if the pin is latched in place (I manage to change on one loco and the problem remains
- I never observed a remanent magnetism that could explain malfunction but rather the moving element 'which is very light' getting stuck and not going down all the way
Below is a picture of an open Telex coupling (bottom plastic part removed) where you can see the moving element
If it has been lubricated or moisted with any liquid it needs to be removed and cleaned carefully with isopropyl-alcohol.
Open only if you are familiar with dealing with micromechanism

If it has never been opened, it may be stuck and need some "running in" by a program written on the CS2 to activate and release it periodically. The pin must going up and down every time. I have 20 of these couplings and most and working very fine

Cheers

Jean

On the plus side, there is a new telex design that is part of the special insider model #1 of 2018, and we can hope that this new design (in addition to being smaller (no lump) is longer term reliable, and some time over the next years we can replace the older design telex couplers with the new types.