Apologies, the message is 'decoder was read' again, on endless loop. Many thanks.

One of the issues with mfx decoders is that the CS3 insists on using it to communicate with a loco if the loco has it. Possibly your Roco loco does and the CS3 is insisting on using that feature. You have three choices - possibly. The best one is to delete the dcc registration you created in the CS3 and allow the mfx registration to be complete. The second is to turn off the mfx feature in the CS3 but then you will possibly lose registration of any other mfx registered locos you have in the CS3. And finally, some non Marklin locos supposedly have the ability to turn off the mfx in the locp. If your loco has that and you can figure out how to turn it off then the CS3 will ignore the mfx handshake with the loco and the dcc will prevail. Moat do not understand that fx, mm, dcc, mfx is a communications agreement between the computer chip in the CS3 and the loco and mfx is simply the chios in the CS3 and the loco agreeing on what frequency they will use to communicate with in privacy.
However, I just remembered that we had a similar occurrence with the CS3 on our club layout and I can't remember if we solved it or it just went away.

Thinking some more about the problem and the fact that it went away on the club CS3 where the programming track is never used and most of the locos in the CS3 are owned by members so rarely ever on hand and finally the fact that the user and probably the CS3 are new makes me wonder if an update to the CS3 fixed the problem and the new CS3 has not been updated. Perhaps the solution is to update the CS3. The current version of the software is 2.5.2(6). What is the software version in your CS3, Copenhagen?

Thanks for your responses. I also took advice from my kids who suggested asking co-pilot. It came up with this. I'll try later today. It looks plausible, although I find with AI, nothing is really as it seems! Will let you know if it works.

Regards

Cameron

This particular locomotive uses a Zimo‑based PluX22 sound decoder, and that combination is notoriously problematic with the Märklin CS3’s DCC “decoder read” routine.
Let me walk you through what’s happening and how to fix it cleanly.
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What’s inside your Roco 78191
According to the official documentation for the 78191, it uses a PluX22 digital sound decoder (Zimo OEM)
These decoders have two characteristics that matter here:
1. They send a weak or non‑standard DCC ACK pulse
The CS3 expects a very clean, strong DCC acknowledgment when reading a decoder.
Zimo OEM decoders often send:
a delayed ACK
a weak ACK
or no ACK at all
The CS3 interprets this as:
“I didn’t get a proper response — I’ll try again.”
→ Endless loop
→ Track signal interruptions
→ Locomotive runs badly
2. They do NOT support mfx or Märklin’s pseudo‑readout
So the CS3 keeps interrogating the decoder, waiting for a response that will never come.
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Why your 78191 does not run properly
Every time the CS3 tries to “read” the decoder:
it briefly cuts the DCC track signal
the Zimo decoder loses commands
speed steps are missed
sound functions glitch
the loco may stop or surge
This is exactly the behaviour you described.
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The fix (and it will NOT affect mfx auto‑registration)
You can keep mfx auto‑detection ON for your Märklin locomotives.
You only need to disable the CS3’s DCC decoder readout, which is separate.
Step 1 — Turn off “Automatic decoder readout”
On the CS3:
System
Loco Configuration
Disable:
Automatic decoder readout
Automatic CV read
Read decoder on registration
Step 2 — Delete the faulty entry
Remove the 78191 from the loco list.
Step 3 — Add it manually
Add locomotive → Manual
Select DCC
Enter the address printed on the Roco box (usually 3 or 10)
Select 128 speed steps
Save — do NOT read the decoder
Step 4 — Test the locomotive
It should now:
run smoothly
respond instantly
play sound correctly
stop the “decoder was read” loop entirely
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If it works you need to tell us what specific steps you took and how to find the location to execute them. For example where exactly on the CS3 and how to disable "Automatic decoder readout", "Automatic CV read", and "Read decoder on registration".

The Mobil Station is better than the CS3 in accessing, reading and changing any odd CV in DCC decoders in my experience.
But changing the locomotive address on DCC decoders should be fine and easy on the CS3. It's just editing the locomotive, changing the address from 3 to another number and save (or click OK)... on the programming output and track to be safe.
I don't think I even use "the read decoder" function.