If you move your Sub to a different spot in the room, do you have the same problem? Are you able to connect your Arc directly to the router with an ethernet cable? What router are you using?

I have not tried moving the Sub, it’s only 1 m away from the Arc. Also, I have not tried using a cable. I have a One which is even further away, and working flawlessly.

I’m using an AVM FRITZ!Box, the Arc and Sub are connected to an AVM Repeater (which is connected to the Router via cable and acts as access point).

I had hoped that the diagnostics might help to identify the problem...

I think the Repeater might be causing the problem. Can the Arc be connected to the Fritz!Box rather than the Repeater? Is any Sonos speaker you own close enough to the Fritz!Box where it can be connected directly to it with an ethernet cable?

Note that the SUB connects to the Arc, rather than the signal from your router.  It uses a 5Ghz channel, so it may be worth looking at making sure that the radio/wifi is still turned on on your Arc in the Sonos controler’s room settings, and that there aren’t any potential sources of wifi interference that might impact the 5Ghz signal. 

The ? is an indication that the SUB isn’t properly connecting, so there’s other things beyond the previous paragraph, and the suggestions from @GuitarSuperstar has provided. It might be worth refreshing your network, too, by rebooting your router. 

And I concur with the concern about the access point, in general Sonos doesn’t like those types of things, and most often recommend a wired network setup, where one Sonos device (not a surround or SUB) is wired to the root device of the router. Access points frequently split up networks, and make it hard for Sonos devices to communicate across the split, even though they frequently have the same SSID and password. 

I connected the Arc via ethernet now. Let’s hope this will fix the issue...

I recently have encountered a similar issue with an arc and a beam on different floors both with individual subs.  In my case they say error not connected, do you want to configure or something to that affect. I have found rebooting the sub does nothing, but powering the sound bar or both seems to fix issue. I have had both sound bars for close to a year with no issues until last month or two. With that in mind I believe the issue is a big on the Sonos side of things. My subs are very close to their respectable sound bars so that can’t be the issue. I have not changed my WiFi either and have very good signals in my house so that shouldn’t be an issue either. My guess is this issue stems from a couple updates back until then I never had issues. It does aggravate me though at random no bass. 

It isn't a bug. There would be thousands of posts about it if there were. Everybody blames a bug. It hardly ever is. 

What sort of router are you Using? Any access points or extenders?

Of course I have access points Sonos main selling point is WiFi.  I have access points on every floor of house built into the ceiling. They are not extenders but fully wired access points that offer the same ssid but are tuned to not broadcast through the floor to help the WiFi rfc figure out the best connection as all WiFi does. . As stated I don’t have issues with other speakers and I have at least 9 Sonos speakers. Also from sub to sound bar does that use proprietary connections or does it use WiFi?  All I can say is I have not had this issue with my previous sound bar, which I had for years or my 1 year ish old arc and beam. They both twice in last 6-8 weeks have exhibited this issue and I do wonder/suspect it came after an update.  Today rebooting the sound bars was the fix needed for whatever reason. 

Side note both times this has happened both subs go offline as a pair. I will monitor the access point the bars use but nothing else is having issues on my network. 

Hi. I too recently started having this problem with an Arc and Sub. It’s been working fine for months and suddenly this has started happening. I’ve changed nothing.

I have factory reset the sub and reconnected it. It worked for a day and now the ? appears in the app.

Do you know if the Sub is still working, even when you see the ? in the App - can you try playing music audio or a 5.1 TV audio source just to see if outputs the .LFE channel. Also what is its status LED showing (assuming you have left it enabled in the HT Room Settings)?

I’ve been experiencing this same issue recently as well. The only (temporary) fix I have found is rebooting or resetting the Arc and Sub.  Didn’t have this issue before.  I suspect it’s a software issue.  

I suspect you have a network issue, either wifi interference inthe 5Ghz range, which the Sub connects to the Arc on, or less likely, but still possible, a duplicate IP address issue in your router. 

If it were a software issue, there would be thousands of posts. 

Why do you say it uses a 5GHz channel when the Sonos app lets you choose the channel and only 1,6 and 11 are available - no 5GHz channels?

Because that’s the channel that Sonos has set up for all sound bars they make to communicate with Surrounds, and Subs. It’s a shorter distance frequency, and since all devices should be in the same room, there’s no issues.

The channel you choose in the controller is for ‘normal’ speakers, including the sound bars, to connect to the router. Surrounds are not ‘normal’ in that setup, since they are bonded to the sound bar. Note, however, that this decision was taken years and years ago, when the PLAYBAR was released. It simply hasn’t changed since that time, for better or worse. I suspect at time of development, there was no expectation that eventually you’d be able to use the 5Ghz channels for ‘normal’ connections, but that’s just a guess. 

I have exact same issue

As mentioned earlier in the thread, a Sub, when bonded to a player, uses an ad-hoc 5Ghz wireless connection ‘direct’ to that player… so I would check the wireless adapter is switched ‘on’ on that controlling device and reboot the player to re-negotiate their connection.

If the problem persists, perhaps move the Sub closer to the main player, or check to see if anything is perhaps getting in the way of that ‘direct’ connection  - baby monitors, other wireless devices walls, beams, etc; can often interfere with the connection, just as examples.

I fixed the problem of the ? By going to support and hitting “Find Missing Products” 

this was all after resetting the Sonos devices and the router dozens of times. Once you pair the Arc and sub again, unsuccessful probably, then go the Support/Find Products route 

Ignore my message above. The same thing keeps happening over and over again. The sub will not stay connected to the Arc. I would absolutely not recommend Sonosto anyone. This is the most expensive pile of sh*t I have ever purchased. Stick with Bose. 

Sorry to hear that news, It may be best if you can submit a system diagnostic report (whilst the sub is showing in the Sonos App) and post its reference back here and then contact Sonos Support Staff via this LINK

Hope the matter gets resolved soon.

Do you have Sky Q?

I have been suffering from the same SUB/AMP disconnection issue. Very frustrating. I just finished up a call with Sonos support and was told that if you are specifically pairing the SUB with an AMP, “the SUB has to be within 10 feet of the AMP for them to talk to each other properly”. That was a direct quote from the support representative. I explained to him that my actual speakers are 30 feet away in the next room so I put the sub in that room so you can hear the base in the same room as the music thinking it would work. He said it will not. Frankly, I’m a little shocked that they do not disclaim this serious flaw when they are selling the AMP. Sending all of my new products back and will revert to the old S1 system until Sonos addresses this issue. Hope others have found better results.

The Sub uses a wireless 5ghz connection to the Amp. 5Ghz is a high speed connection, but is more sensitive to interference. I can understand placing the Sub in another physical room (maybe even separated by a wall?) than the Amp can lead to problems.

I can understand your frustration, but do find your use case a little specific. If you have a more specific use case, I would expect a customer to read up on the possible problems to expect. The use of a dedicated 5Ghz link between Amp and Sub, and so the possible problems this would pose to your use case is one of the things you’d have known about.

Have you spoken to Sonos about connecting the Sub in a wired configuration like this:

Seeing your remark about disclamers. I think you expect too much. If Sonos would disclaim this, what specific use cases would they have to disclaim further?

Since in your use case Sonos might not be an option, it is good there’s a return policy.

I am still having the issue it started in spring. I have arc,beam and two subs. The sound bar and sub pairs are on different floors. When the issue happens both subs go offline together every time. They may or may not be connected to same WiFi access point as I have more than one. The sound bar subs are a few feet away from sound bar. The only way I can fix it is by rebooting the sound bars. I am struggling to believe this is WiFi related, nothing else in my house including 7 other sonos speakers have issues. 

Might not be WiFi, could be network, though.