How to fit a 21MTC, PluX or Next18 decoder
Identify which modern direct-plug interface your loco has, choose a matching decoder and fit it without cables or soldering.
- Time
- 30 minutes
- Difficulty
- Easy
- Cost
- Moderate cost
- Steps
- 6
- Checked
What you’ll need
- A decoder with the matching interface
- Small screwdrivers Search Amazon for precision screwdriver set
- Tweezers Search Amazon for fine tip tweezers
- Magnifier Search Amazon for magnifying head lamp
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Before you start
- Disconnect power before working on the loco.
- Never force a decoder onto pins; misaligned pins can bend and short.
Step by step
-
Step 1: Identify the interface
- 21MTC (NEM 660): a two-row strip of 1.27 mm pins on the loco board with pin 11 missing as a key; decoder up to 30 × 15.5 mm.
- PluX22 / PluX16 (NEM 658): sockets on the loco board, pin 11 omitted on the decoder as a key; PluX22 decoders up to 30 × 16 mm, PluX16 up to 20 × 11 mm.
- Next18 / Next18-S (NEM 662): an encapsulated 18-pin socket for small N, TT and HO installs; Next18-S is the longer sound version. (NMRA S-9.1.1.3, S-9.1.1.4, S-9.1.1.5)
-
Step 2: Check the loco's own instructions
ESU notes that some Märklin 21MTC locos and some locos sold as PluX do not follow the standard, and need specific decoder variants. Check the loco maker's advice.
Next: step 3 -
Step 3: Remove the body and the blanking plug
These interfaces plug straight onto the loco's circuit board; there is no wiring harness (NMRA). Keep the dummy plug.
Next: step 4 -
Step 4: Fit the decoder the right way up
For 21MTC and PluX, the missing pin 11 / blocked socket stops it going on the wrong way. Next18 contacts are arranged symmetrically so a rotated fit should not cause damage, but fit it in the intended orientation (NMRA S-9.1.1.5). Press gently and evenly.
Next: step 5 -
Step 5: Connect the speaker if it's a sound decoder
PluX22 and Next18-S include speaker connections (NMRA). Some locos have the speaker wired to the board already.
Next: step 6 -
Step 6: Test on the programming track
Read CV1 and CV8, then test running and lights on the main.
Tips
- Contacts on these connectors are rated at about 1 A (21MTC, PluX) or 0.5 A (Next18) each (NMRA) – fine for typical small-scale motors.
- The newer NMRA E24 interface (2026) is not keyed – take extra care with orientation if you meet one.
Questions people ask
Is PluX16 the same as PluX22?
No. PluX16 uses only pins 3 to 18 of the same layout and has a smaller size limit (NMRA S-9.1.1.4). Check both the loco and decoder instructions before mixing them.
Which is best for N gauge?
Next18 was designed for small installation spaces such as N and TT (NMRA S-9.1.1.5); the loco's socket decides.
Sources
Checked 18 October 2026. We wrote this guide in our own words, using:
- NMRA S-9.1.1.3 21MTC Decoder Interface (nmra.org)
- NMRA S-9.1.1.4 PluX Decoder Interface (nmra.org)
- NMRA S-9.1.1.5 Next18 / Next18-S Decoder Interface (nmra.org)
- NMRA S-9.1.1 Decoder Interfaces (nmra.org)
- ESU LokPilot 5 instruction manual, Edition 7 (distributor copy) (intermountain-railway.com)
- MOROP NEM standards list (morop.org)
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Printed from dcctrainautomation.uk/guides/fit-a-21mtc-plux-or-next18-decoder/ · Checked 18 October 2026 · General information only. Follow the maker’s instructions for your own equipment, and keep mains wiring inside properly made, enclosed power supplies.