I should add, when sitting with the iOS app open I don’t see any changes when this happens, and equally I don’t see anything unusual in ‘about my system’.  For connectivity, I have a Sonos Boost to which everything is connected, so I wouldn't naturally suspect anything wifi/connectivity-related.

At the time this is happening, we’re watching TV via the Sky Stream box.  There is no obvious interruption in picture or sound when it’s occurring.

Sure sounds like a failing LED….and if it doesn’t impact the sound at all, I’d ignore it. Although I’m one of those people who turn off the LEDs in the controller all the time. 

Am I correct -- this is a brand new ARC? If so, I would take this up with SONOS support.

Wait for it to dim, send a diagnostic and contact support with that number. If nothing else it will document your problem in case things get worse later.

Thanks all.  Will get a diagnostic asap.

I haven’t managed to find this in Sonos’ own documentation, even though I am sure I recall seeing it.  But here is something from a What HiFi review.  I wonder if what you are seeing is normal behaviour?

The Arc's own status light consists of LEDs that shine through four holes above the subtle Sonos logo (fun fact: there are 76,000 holes in all the grilles combined). By default, these LEDs remain off most of the time and their brightness is tailored to the ambient light in the room even when they are lit (when adjusting volume, for example), so as not to appear too bright.

Thanks, that is interesting. However, we've manually enabled the status led via settings so it remains on whilst we try to work out the other issues. It's unlikely to be the ambient light levels as the room is quite low in natural light anyway, and wasn't changing at the time. 
 

Will continue to monitor in any case.

This doesn’t make sense.  If the status light is disabled in Settings, it is off.  If enabled, it dims in low light.  So most (all?) of what you say is actually consistent with my suggestion, while you seem to believe it contradicts it. The status light on my Arc is enabled, but is not lit at all when the room lights are turned down for us to watch a film.  This stops the status light being intrusive in low ambient light, which is the purpose of this feature. 

I’m confused.  The status light on our Arc is enabled, and it displays all of the time - I don’t believe I’ve ever seen it turn off.

The original issue I reported is that I can visibly see the status light doing from ‘bright’ to ‘dim’ whilst watching TV in the middle of the day with consistent ambient light in the room.  I’m trying to understand *why* it would otherwise be changing in brightness, as to whether that implies some issue either with the soundbar or the components it’s connected to.

The only way I can see what you’ve suggested causing what I’m seeing, is that the ambient light level is somehow ‘on the threshold’ of what causes the status light to dim, and that it’s toggling back and forth. I very much doubt that is the cause of what I’m seeing.

There is probably some hysteresis in the system so that the light isn’t constantly changing.  But with the status light enabled, if the room light is very low or completely darkened then the status light will fade and go out. At least is does on my Arc.

I am not saying that your light is definitely working correctly - I have no way of knowing that.  I am just saying that I believe the light is designed to go out in low ambient light and that I have witnessed that behaviour on my own Arrc.

If the unit is sensitive to ambient light, you should be able to block the sensor and trigger the intensity change. I’d start with a large sheet of cardboard and cast a shadow. If the intensity changes, you can move the carboard and locate the light sensor. Maybe it is in the infrared window.