Last updated: 19 October 2025 (content)
The MOST loop test is a diagnostic method that allows you to confirm conclusively whether a fault (e.g. freezing on the logo or restarts) in a Mercedes with the NTG2 system is caused by a failure of the Comand unit itself, or by another module in the fibre-optic ring (e.g. the amplifier or changer).
Your Comand NTG2 radio in your Mercedes freezes on the logo. Sometimes it restarts, sometimes it briefly comes back to life, only to go dead again a moment later. You are faced with a frustrating dilemma: what is actually broken? Is the radio really at fault?
Replacing modules “blind” is an expensive gamble. You could spend money on a used radio only to find that the problem lies elsewhere (for example in the amplifier). Before you make any service decisions, you need 100% certainty about which component is failing.
In this DIY guide, we will show you a simple 10-minute test that you can carry out yourself using a tool costing just a few dozen złoty. You will confirm whether it really is the Comand that is faulty. If so, you will immediately learn the standard refurbishment conditions for such a unit (the cost is usually 984 zł incl. VAT, repair takes 1-5 days, and the warranty is 12 months).
Step 1: Which symptoms qualify for this test?
This guide is for you if you are experiencing any of the following problems in your Mercedes, indicating a break in the fibre-optic loop:
- Stuck on the Mercedes logo (Boot Loop): The radio starts up, shows the “Mercedes Benz” logo and stops there, or repeatedly restarts in a loop.
- Random restarts while driving: The radio works for a while, then suddenly switches off and tries to start again.
- Battery drain: Sometimes the fault is accompanied by a constant current draw, even after the ignition is switched off, which drains the battery overnight.
Step 2: Which devices does this guide apply to?
This method applies to the Comand NTG2 navigation system (manufacturer: Harman/Becker), fitted in Mercedes vehicles from around 2005 onwards.
You can find these units in the following models, among others:
- Mercedes A Class (W169)
- Mercedes B Class (W245)
- Mercedes C Class (W203)
- Mercedes CLK (W209)
- Mercedes ML (W164)
- Mercedes GL (X164)
- Mercedes R (W251)
Step 3: Understanding the problem – how does the MOST bus work?
Before you start the test, you need to understand why the diagnosis is so important.
In your Mercedes, the individual audio modules (Comand radio, changer, phone control unit, amplifier) communicate with each other via fibre optic. The whole system works on the principle of a ring (MOST bus).
Imagine data flowing from one module to the next, creating a closed circuit. If any module in this ring is faulty (for example the amplifier), the data is not passed on to the next one. The circuit is broken (open).
The end result of a broken ring is precisely such anomalies as the navigation freezing. That is why the Mercedes logo on the screen does not always mean a fault in the Comand NTG2 radio itself. Another module in the loop may be to blame.
Step 4: MOST loop test [STEP-BY-STEP INSTRUCTIONS]
The quick way to check whether your Comand really is faulty is to carry out a test on a closed fibre optic loop.
This involves disconnecting the original fibre-optic cable from the radio and connecting a special MOST loop (jumper) in its place. It is nothing more than a piece of “bent” fibre optic cable that “tricks” the system into closing the ring without the radio.
Fault verification procedure
The test consists of the following steps:
- Remove the Comand NTG2 unit from the dashboard.
- Disconnect the main power connector (this includes the fibre optic lead).
- Unplug the small double fibre optic connector from this plug (usually orange wires).
- Plug the previously purchased MOST loop in its place.
- Reconnect the modified power connector to the radio and switch the unit on for testing.
Interpreting the test results (key moment!)
You now have two options:
- Comand IS WORKING: If the unit works correctly at this point - you can see the full menu, switch between radio stations, and nothing resets or freezes - then your Comand NTG2 is working properly. You will find a full repair description in the article COMAND NTG2 repair. The fault must be looked for in other MOST ring modules (for example, the amplifier or changer). The only anomaly in this mode will be the lack of sound (because you have bypassed the amplifier in the loop - of course, if your car has an amplifier).
- Comand IS DAMAGED: If the problem still occurs (even with the loop fitted, the radio still hangs on the logo or restarts) - you have 100% confirmation. Your Comand NTG2 is faulty and can be repaired.
Step 5: Diagnosis confirmed - what next?
If your test clearly identified the Comand unit as the culprit (scenario 2), you no longer need to look “blindly”. You can commission a refurbishment of the damaged unit.
Effective repair involves refurbishing the unit's main board. At OEMatic, the process looks like this:
- Total cost: 984 zł incl. VAT
- Lead time: 1-5 working days
- Warranty: 12 months (written warranty - VAT invoice)
The repair is carried out at hardware level, which means that once the unit is returned it is fully ready to fit (plug & play) and requires no coding at all.
Detailed description of the cause of the fault (why does NTG2 fail?) and the refurbishment process step by step can be found in our main service article:
➡️ [Comand NTG2 Mercedes repair – hangs on the logo? How do we refurbish it]
About the author of this article
Prepared by the workshop's technical team. We specialise in advanced diagnostics and repair of factory radio-navigation systems from premium brands. We have many years of experience and hundreds of repaired Comand NTG2 units fitted in Mercedes Benz cars to our name.
Need help with diagnosis or repair?
If, after reading the guide, you still have doubts or your diagnosis confirmed a fault with the Comand - we are at your disposal.
