Suddenly almost always no signal with Xperia Z5

Thanks for your reply. I haven't tried the SIM in an other phone lately (I will now), but at least it's always recognised and it never shows directly a SIM-card error/failure.
The previous owner of the phone told me, that the display (or the touch screen, i don't know) has been replaced, so it suffered a mechanical damage some time ago...

Edit: I just put the SIM card in an other phone - for now it seems to work just fine there. Vice versa a other SIM (other carrier) in the Xperia Z5 also seems to get no connection to the mobile net :/

Edit 2: I should add, that, since I own it, the phone always seemed to have some fluctuations in the network strengh, even if you don't move (from full strengh to one or zero bars), but I was always able to use it and it never lost the connection to the mobile network completely like now.

Yes, there is one blue wire. It is for the radio signal you say?! The connections of it seems tight. Should I be able to measure a, e.g., to high resistance or is that no valid criterion?

Thanks for your reply.

svim said:

Just note that a smartphone antenna, which Sony refers to as a 'signal cable', is just a piece of wire.

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Okay, there is this blue signal cable, but there are also those other two so called antennas I linked in the first post (see pictures) (and an additional WLAN antenna) Does someone know which antenna(part) is needed/used for what?

It doesn't have any internal electronic components, (...) The odds are that little piece of wire actually 'fails' is unlikely.

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As I mentioned in the first post, I think/thought so aswell for those two antennas. They seem to be quite simple. That's why I asked

Have you also been having issues with Bluetooth? A lot of phones share the WiFi and Bluetooth antenna.

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No, WiFi workes completly fine. I usually never need bluetooth, but a short file-transfer-test from phone to phone worked also fine. It's only the "mobile" GSM/EDGE/LTE/etc. network connections that don't work.

There are some repair guides for the Z5 you might want to look at, if you care to actually go through the steps to re-seat those signal cable connectors

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Thanks, I already re-seated the blue wire - with no change. I also couldn't notice some kind of loose connection, they seem tight :/

but I'd think you'd want to focus more on the SIM card as that would be more likely to be an issue.

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Im fine with everything, what would you suggest that I can try more? As mentioned, the SIM card itself workes fine in an other phone.

Edit
So, let's get a bit more confusion into this topic: Yesterday most of the time the problem was, that there was no connection at all. It was not "zero bars" (that means greyed out bars) in the network-meter, there were no bars (an empty triangle).
Now, at this moment I have all four bars, a signal strengh of -89dBm, so quite good, the right carrier, but at "mobile network status" it says "not connected". Those two problem descriptions don't really fit together :/

Thanks once more for your post. I'm quite confused now, because it's different to yesterday. I will answer your questions one by one:

svim said:

If you haven't experienced issues with WiFi and it's just cellular connectivity, odds are this isn't an antenna issue

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Positive, no problems at all with WiFi, only with cellular connectivitiy. But there is a dedicated WLAN antenna, so isn't there a possibility, that the WiFi antenna is fine and only (one of the) "cellular" antennas has a problem?

and since the SIM card works out in a different phone that's probably not the source of the problem either.

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I just put the SIM card once more into a other phone and called a third one. Everything worked fine, also the internet connection per mobile data. So the SIM card and the cellular connection works fine with an other phone.

You're referring to issues with phone calls so just to confirm that's not WiFi Calling related but just typical phone calling through your carrier's cellular connection, yes?

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That's right, a typical phone calling through my carrier's cellular connection

If you disable WiFi are there any connectivity issues using just mobile data?

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Same as the telefone calls, so no Internet connection then.

or just some kind of issue with the signal reception in general (i.e. something affecting the cellular signal broadcast range to the closest cell towers). Where are you located and which carrier service do you use?

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I'm here in Vienna, Austria and had the problems in different parts of the city. The carrier is HOT and they are using the network from T-Mobile.

Since this problem started a couple of days ago, can you think of anything that might have occurred around that time? Applying an update to your phone? That might have brought the problem up. Or did some new building or construction project just start up nearby? That might be affecting local cell towers or their broadcast coverage.

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No system updates and nothing else i could remember. I used a heat gun to loose the glue of the backcover, but that was after I had the problems.

At the moment, I have no cellular connection at all, so Opensignal also doesn't show anything. An hour ago I had a good signal but the phone wasn't connected to the network...

Hmm, I'm confused, why do you think so when it works with the same SIM card in the other phone? But no, i have not contaced them till now...

Thanks once more to all. I already measured the resistance of the wire: <1Ohm, so the wire itself isn't broken. I was able to buy the two mentioned antennas out of a phone with a broken mainboard for 8 Euro shipping included and hope for best. The seller told me, that the phone never had issues with the cellular connection. I try not to be too optimistic, but it's worth a try. If that dosen't change a thing, I will reflash Android and afterwards I'm quite out of ideas, except the mentioned cellular chip failure.

How I observed the problem the last days, the phone gets a strong GSM signal from time to time (like in the screenshot above), when I set it to "GSM_only" in the service settings, but it still does not get connected to the network. If that's the case, OpenSignal shows a 2G connection. And the rest of the time it has a signal strengh of zero.