Mercedes COMAND NTG2: the display and steering wheel work, but the radio buttons do not. How can you check whether MOST is to blame?

Key information: COMAND shows the normal menu, plays sound and responds to volume, station or track changes from the steering wheel, but does not respond to the buttons on its own panel? Do not send it for refurbishment straight away. This symptom pattern is a strong clue pointing away from the unit itself. First, this needs to be confirmed with a fibre optic MOST network test.

First, let’s sort out the names: radio, navigation, COMAND and NTG2

Mercedes owners often simply say “radio” or “navigation”. In this guide, we mean the factory Mercedes-Benz multimedia system COMAND NTG2.

• COMAND is the name of the system combining, among other things, the radio, navigation, telephone and control of other multimedia devices.
• NTG2 means the system generation: New Telematics Generation 2.
• The COMAND panel refers to the buttons and knobs located directly on the unit.
• MOST is the in-vehicle multimedia network through which COMAND communicates with other modules.

This guide deals with a specific pattern found in vehicles with COMAND NTG2. It should not be applied automatically to every Mercedes radio or every problem described as “freezing up”.

The shortest answer

The key pattern looks like this:

normal menu + working audio + dead COMAND keypad + working steering wheel controls

A COMAND like this does not behave like a unit that has frozen during startup. It still produces an image, plays the selected source and carries out at least some commands coming from the car.

In practice, at OEMatic with exactly this set of symptoms, the first area to check is the MOST network, especially the telephone module. However, this does not mean you should buy a telephone module “blind” as a trial. First you need to:

1. confirm the full symptom pattern;
2. separate COMAND from the other MOST devices;
3. check whether its panel has regained control;
4. if so — restore the system and bypass individual modules, usually starting with the telephone;
5. for the indicated module, also check the power supply, ground, connector and fibre optic line.

The loop does not fix the fault. It allows a controlled experiment and answers the question of which side of the system contains the cause.

A clue versus confirmation — an important distinction

There are three levels of certainty in this diagnosis:

This distinction protects against two mistakes: unnecessary refurbishment of a working COMAND and a rash purchase of a phone module without checking the wiring.

What can you check yourself, and what should you leave to an auto electrician?

Without dismantling anything, you can

1. Start the system and check whether it goes to the normal menu.
2. Check a few sources, e.g. radio, disc or AUX, if available.
3. Press a few different panel buttons: Radio, Navi, Telephone, settings and disc eject.
4. Change the volume from the steering wheel and confirm that the sound level really changed.
5. Try changing the station or track from the steering wheel.
6. Note whether the symptom depends on temperature and how long it takes to return after a reset.
7. Record a short video showing the screen, the attempt to use the panel and the steering wheel operation.

The auto electrician should carry out

1. STAR/XENTRY or DAS diagnostics before clearing faults and disconnecting the system;
2. checking the power supply, grounds, connectors, fibre optic cable and signs of moisture;
3. correct isolation of COMAND from the rest of MOST;
4. single bypassing of subsequent devices;
5. comparing results always carried out in the same way;
6. a decision to repair only after identifying the actual fault area.

If you do not know how to safely remove the unit or disconnect the fibre optic connector, end the self-test by recording the symptom and pass the further instructions to the auto electrician. It is not necessary to strip the device down to carry out a network test, but you must gain proper access to its connector.

What is MOST?

MOST stands for Media Oriented Systems Transport. It is a network designed to transmit multimedia data and commands between vehicle devices. In the Mercedes NTG2 described here, it uses fibre optics, and the devices form a closed loop called a ring.

The easiest way to picture it is as a relay. The signal passes through successive devices and returns to the starting point. COMAND therefore does not work in complete isolation. It works with the other participants in the system.

Depending on the equipment, these may include:

The full list and order of devices depend on the model, year, factory equipment and any previous modifications to the car.

Why can one module affect COMAND?

The telephone module does not have to be completely dead or physically break the fibre optic cable. It may carry out some functions and at the same time, after a while, take part in the system communication incorrectly. That is why a working phone call or Bluetooth do not rule out a problem in this area.

Other causes related to the same area are also possible:

• unstable module power supply;
• poor earth;
• moisture or corrosion in the connector;
• a fibre that has come loose or been damaged;
• previous modifications to the wiring;
• an intermittent fault appearing after warm-up.

For this reason, the loop result points to the problem area, but does not always immediately prove damage to the module electronics.

Why does the panel not work, but the steering wheel does?

The COMAND panel buttons and the steering wheel buttons are not one shared set of contacts. Commands from the steering wheel are transmitted through the car's electronics and control network differently from a local button press on the front of the unit. The detailed communication path can vary between models and trim levels, so you should not draw one identical communication scheme for every car without documentation for the specific variant.

The functional observation is the key point:

This is not the typical “the radio is dead” case. COMAND is still working, but its local operation is not functioning correctly.

Why does a reset help only for a moment?

Removing the fuse or disconnecting the battery restarts not only COMAND, but also the devices working with it in the system. A faulty module may work correctly for a while after a restart, then after warming up or when the communication problem occurs again, return to the incorrect state.

That is why the information “after disconnecting the battery the radio works again” does not prove that COMAND is faulty. It only shows that restarting the system temporarily changes the behaviour of the fault.

If the problem occurs intermittently, the loop test should be carried out when the symptom is active. Otherwise, a temporarily correct system may give a misleading result.

What is a MOST loop?

A MOST loop is a fibre-optic link that bridges the signal input and output at the point where the disconnected device sits. This allows the device to be temporarily taken out of the signal path without leaving the circuit open.

Loop:

• does not repair the module;
• does not programme the system;
• does not erase the cause;
• lets you check whether the system behaviour changes after bypassing a specific section;
• it can be used first to split “COMAND or the rest of the MOST network”, and later to locate the specific device.

You need a loop that matches the optical connector used. Do not buy a random part based only on the colour of the housing. The connectors must be disconnected with the ignition off, without pulling on the fibre and without looking directly into the active end of the fibre optic.

First test: COMAND or the rest of the MOST?

The aim of the first test is to leave the running COMAND in a locally closed optical circuit and disconnect all devices further down the MOST from it.

Watch out for the reverse test: this is not about bypassing COMAND in the car's wiring. If the radio is removed from the signal path, its panel cannot be checked. The loop should be fitted at the unit so that COMAND is working without the rest of the devices.

How do you carry out the test?

1. Cause the fault to appear and record the system behaviour before disconnecting.
2. Switch off the ignition and allow the devices to go into standby.
3. Access the appropriate COMAND optical connector according to the documentation for the specific model.
4. Keep the connections needed to start the unit, and close the optical path locally with the correct loop.
5. Start the system under the same conditions as before.
6. Check the Radio, Navi, Telephone and settings buttons, both knobs, menu navigation and disc ejection.
7. Do not judge the test result solely by the sound.

You will find a detailed description of how to do this in a separate post: How can you confirm a COMAND NTG2 navigation fault in a Mercedes yourself?.

What should you expect?

• the external amplifier may be disconnected, so there may be no sound;
• the phone, Bluetooth, changer or voice control may be unavailable;
• messages about unavailable functions should be expected;
• the key result is the behaviour of the local panel and the COMAND menu.

How do you interpret the result?

The panel started working: COMAND can operate its own keypad when it is not communicating with the rest of the MOST network. The cause should be looked for outside the unit: in another module, its power supply, the connection or a section of fibre optic cable.

The panel still does not work: the test did not confirm a fault in the external part of the MOST network. Check whether the loop was fitted correctly, then go back to the power supply, the vehicle communication, the panel and the unit itself.

Second test: which module is causing the problem?

If the first test restored panel control, you need to recreate the normal connection at the COMAND and then bypass individual devices one by one.

Correct sequence

1. Restore the normal connection at the COMAND and check whether the fault has returned.
2. Locate the phone module.
3. Disconnect only that module and close its place with a loop.
4. Start the system and repeat the same panel test.
5. If the symptom does not disappear, reconnect the phone and check the next module.
6. Depending on the equipment, move on to Linguatronic, the changer, the amplifier and the other devices.
7. Change only one item at a time.

Record the results

After each bypass, the system should be started up in the same way, given a similar amount of time and the same functions checked. If the fault depends on temperature, it is worth comparing the system under similar conditions.

Full procedure for the auto electrician — seven points

Below is the complete procedure that the owner can pass on to the electrician. It deliberately repeats and organises the earlier explanation in one place, so the workshop does not have to piece the instructions together from several parts of the article. Each point explains what to do, how to do it and how to interpret the result. The first loop test can be started without knowing every module, but a full equipment list will be needed for further fault-finding.

Point 1. Confirm the exact symptom pattern

How to do it?

Start COMAND and check separately:

• whether the screen shows the normal menu, not just the Mercedes-Benz logo;
• whether the radio or the track being played works;
• whether all front-panel buttons are unresponsive, or only some of them;
• whether volume adjustment from the steering wheel really changes the sound level;
• whether you can change radio stations or tracks from the steering wheel;
• whether removing the fuse or disconnecting the power only brings a temporary improvement;
• whether the symptom depends on temperature or operating time.

What does the result mean?

Normal menu, working audio and steering-wheel commands show that COMAND is still operating. This is a strong clue pointing to MOST, but not yet final confirmation. If only the logo remains on the screen and the steering wheel also does not work, there is a different type of fault and the unit itself must be considered.

Point 2. Carry out STAR/XENTRY or DAS diagnostics before disconnecting the system

How to do it?

Carry out a quick full-vehicle test and save the report before clearing faults and before disconnecting devices. Check COMAND, the telephone module, amplifier, changer, Linguatronic and the other multimedia components detected. Note:

• which modules respond;
• which modules there is no communication with;
• where MOST, power supply or wake-up faults are stored;
• whether the diagnostics indicate a system communication problem;
• whether the faults are current, stored or intermittent.

Why before disconnecting?

Unplugging the fibre optic cable or a device will itself create new communication faults. A report made beforehand shows the car's original condition. After the tests, it can be compared with the faults created during controlled disconnection.

What does the result mean?

No communication with the telephone module is an important clue, but it does not yet prove damage to its electronics. The cause may also be a lack of power, ground, connector or fibre optic cable. Likewise, the absence of a stored fault does not rule out an intermittent fault that was not present during the reading.

Point 3. Determine which modules are actually present in the MOST ring of a given car

The equipment differs between models and versions. However, you do not need to know the full list before the first split of “COMAND or the rest of MOST”.

The simplest sequence:

1. Fit a loop at COMAND and isolate the entire rest of MOST.
2. If the buttons start working, the problem is outside the radio.
3. Restore the ring and bypass the individual devices — first the telephone module.
4. If that does not help, check Linguatronic, the changer, the amplifier and the remaining components.

How do you determine the full equipment list?

• From the equipment card by VIN — shows which options the car left the factory with. Look for telephony, Harman Kardon/Logic7, the changer, Linguatronic and other multimedia functions.
• By a quick STAR/XENTRY or DAS test — shows the devices responding at the time of the check and the stored faults.
• Based on the correct wiring diagram — allows you to determine the factory order and locations for a specific model and equipment level.
• Physically and by loop testing — confirms the actual state: devices removed, added, swapped or previously bypassed.

On the working list, record the name of the device, its location, the communication result, the power supply status and information about earlier repairs or modifications.

What do the discrepancies mean?

• a device present in the factory equipment but not visible in diagnostics requires checking of power supply, connectors and fibre optic cable;
• a device responding with MOST faults is a candidate for controlled bypass;
• a difference between the equipment card and the physical condition indicates an earlier modification, removal of a module or a loop left behind;
• Correct communication does not rule out an intermittent fault — what matters is a repeatable change in system behaviour after bypassing the unit.

Step 4. Check power supplies, earths, connectors, fibre optics and signs of moisture

How to do it?

Before buying or sending in a suspected unit, check:

• the correct fuses and supply voltages;
• the quality of the earth connection during actual module operation, by measuring the voltage drop between the earth at the unit and the negative terminal of the battery;
• the condition of the pins, locks and connector housing;
• moisture, corrosion and signs of previous flooding;
• whether the fibre optic is not broken, sharply bent or pushed out of the insert;
• whether the connector was correctly reassembled after previous work;
• whether there is optical transmission at the correct point, without looking directly into the active fibre.

What does “check the earth while the module is operating” mean?

A simple continuity reading on a multimeter is not enough. A corroded contact or a partially damaged wire may show continuity, yet under real current draw cause a voltage drop and unstable electronic operation. The auto electrician should therefore measure the voltage drop between the earth at the working module and the negative terminal of the battery, in accordance with the wiring diagram for the specific vehicle.

Why?

The unit may not respond because it does not have the correct operating conditions. Fitting a used module into a system with no supply, corrosion or a damaged fibre may change nothing and create the false impression that the next module is faulty too.

What does the result mean?

No supply, poor earth, corrosion or a damaged fibre optic mean the installation needs repairing. Only correct operating conditions and a repeatable bypass result strengthen the suspicion of a fault in the module electronics.

Step 5. Isolate COMAND from the rest of the MOST ring with a loop at the radio

How to do it?

1. Confirm that the symptom is present and record the initial state.
2. Switch off the ignition and wait for the devices to go to sleep.
3. Gain access to the unit's optical connector according to the documentation for the given model.
4. Close the optical input and output at COMAND itself with the correct loop, so the unit operates without the phone, amplifier, changer and other devices further along the network.
5. Take care not to bypass COMAND in the vehicle loom — that would be the reverse test.
6. Start the unit and check Radio, Navi, Telephone, settings, rotary controls, menus and disc ejection.

What does the result mean?

• if the panel started working, COMAND is able to handle its own keyboard and the problem lies further along in MOST;
• if the vehicle has an external amplifier, there may be no sound during the test — that does not invalidate the result;
• if the panel still does not work, first check the test setup, then the power supply, vehicle communication, the panel and COMAND itself.

Step 6. Bypass the remaining modules one by one — start with the phone module

How to do it?

If the panel started working after isolating the rest of MOST:

1. restore the normal connection at COMAND;
2. confirm that the fault has returned;
3. locate the phone module;
4. disconnect only that module and close its position with a loop;
5. start the system and perform exactly the same panel test;
6. if the symptom remains, reconnect the phone and check the next unit;
7. do not bypass several new elements at once.

Why start with the phone?

In practice at OEMatic, with the exact set “normal menu + whole keyboard dead + steering wheel controls working”, the phone module is the most common first suspect. This is a diagnostic order indicator, not proof that allows you to buy the part without testing.

What does the result mean?

A repeatable return of panel operation after bypassing a specific device points to the fault area. You still need to check its power supply, earth, connector and fibre optic section. Only then can you decide whether to repair the electronics, replace the unit, repair the installation or deliberately disable the function.

Step 7. Decide, on the basis of the result, what really needs repairing

The MOST ring is a diagnostic tool, not an automatic repair. Do not leave a device bypassed by chance without informing the owner which functions have been disabled and whether the system requires further configuration.

How do customers describe this symptom?

In reports to OEMatic, a very similar way of describing this fault keeps recurring: the owner sees a working screen, cannot use the COMAND buttons, but can still control the system from the steering wheel. It is this full set of information that is useful diagnostically — not just the phrase "the radio has frozen".

The fragments below have been shortened and anonymised. They preserve the meaning of the customers' statements, but do not contain personal data. They do not mean that the same cause occurs in every car; they show why the test described in the guide must be carried out before repairing the unit.

"The display works, but the radio does not respond to the buttons. Steering wheel control works."

"After a reset everything works for an hour, and then only station and volume can be changed from the steering wheel."

"The screen, sound and discs work, but I cannot select anything or eject the disc."

"After disconnecting the battery it returns to normal, but the problem comes back again."

"You cannot switch between Navi, Radio and Telephone."

"The disc plays, but it cannot be ejected."

"After removing the fuse the system can be restarted."

"The colder it is, the longer it works properly."

Six representative variants

Why is a simple answer of “it’s the telephone module” not enough?

After that information, customers naturally asked:

“I thought you repaired COMANDs.”

“What is the telephone control unit?”

“What should I do now?”

The owner must receive not only the name of the suspected part, but also a logical chain of explanations: why the steering wheel matters and what MOST is. They must also know what the loop checks, why the sound can disappear and which result allows a repair decision to be made.

When might the problem be in the COMAND unit?

The guide concerns a specific combination of symptoms. The cases below require a different path:

The key rule is: compare the full set of symptoms, not the single statement “the buttons do not work”.

What not to do?

Do not buy a used COMAND just because the panel does not respond

If the cause is elsewhere in the phone, fibre optic cable or another module, the second COMAND may behave in exactly the same way. There is also the matter of matching part numbers, coding and the risk of buying a unit with a different fault.

Do not send your own unit for refurbishment without confirming the direction first

Refurbishing a working unit will not fix the problem in the car. The customer will incur the cost of removal, shipping and the service, and after refitting will see the same symptom again.

Do not replace the front panel straight away

If the whole keypad does not work, but the system still responds to the steering wheel controls, MOST must be checked first. Otherwise you may replace a perfectly good panel just because the fault is visible there.

Do not treat disconnecting the battery as a repair

A reset restarts the whole system from scratch. A temporary improvement does not show which component is causing the problem, and it does not remove the source.

Do not bypass several new devices at once

If the symptom disappears after disconnecting the phone, amplifier and changer at the same time, you still will not know which component mattered. At each step, change only one condition.

Do not buy a telephone module based solely on the symptom description

The phone is the first suspect, but the same area also includes its power supply, earth, connector and fibre optic cable. A used part will not repair a corroded connector or a broken fibre.

Do not clear the faults before saving the report

After disconnecting MOST, new faults will appear. Without the initial report, it is difficult to separate information about the original fault from the results of your own tests.

Do not treat the fibre optic light alone as full confirmation that everything is working

The presence of light at one point does not confirm that all devices are working properly, the quality of the entire path or correct communication. It is only one part of the check.

Checklist and video for the workshop

Before contacting an auto electrician or OEMatic, prepare the information below.

How to record a useful video in 20–30 seconds?

1. Show the normal menu or the logo on the screen.
2. Show that sound is playing.
3. Press several different panel buttons and show that there is no response.
4. Change the volume from the steering wheel.
5. Change the station or track from the steering wheel.

Such a video often says more than the phrase “the radio freezes up”.

Do you have this symptom? Choose the right route

You do not have STAR/XENTRY or loop results

Send:

1. car model and year;
2. information on whether you see the normal menu or the logo;
3. information on whether the sound works;
4. a list of functions controlled from the steering wheel;
5. information on whether the reset helps and for how long;
6. a short video recorded according to the guide in the article.

From the video alone we will not decide on the repair, but we will help assess whether the first area to check should be the MOST or diagnostics of the COMAND unit itself.

The auto electrician carried out diagnostics

Send:

1. fault codes and status without the VIN and personal details;
2. a list of detected MOST devices;
3. loop installation point;
4. the panel’s behaviour after each test;
5. the information showing which device was bypassed before the symptom disappeared.

If the test confirms a fault in a specific module, its repair can then be assessed responsibly.

Technical sources and basis for the material

• Microchip — MOST Technology — the general principle of the MOST network and multimedia and control data transport.
• Mercedes-Benz B2B Connect — information on XENTRY — the scope of diagnostics, tests and fault memory reading.
• OEMatic diagnostic practice concerning COMAND APS NTG2 and the MOST network.

The guide describes the diagnostic logic. The exact locations of the devices, the removal method and the connector layout should always be confirmed for the specific model, year and equipment.