Two Xperia Z5's, same firmware, much different signal strength

[Jak6]

I noticed something very strange with an Xperia Z5 i have.  I have two E6653's side by side flashed with the same firmware. One of them will only get 0-2 bars max on 3g+ the other easily has full bars on 3g+.  The one with 0-2 bars on 3g+ takes extremely long to load pages etc, and sometimes it will not load at all even with 2 bars. The APN settings are exactly the same on each phone and SIM card is the same.  So there must be something wrong with the hardware. So i replaced the antena and even the battery in the problematic Z5 but the results are the same.  Im suspecting something on the mainboard itself is to blame becasue i dont think there are anymore parts to replace that are responsible for signal strength. 

Does anyone know if there are any other parts on the mainboard that effect the signal that could be replaced / soldered?

[Kycpokemon]

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@Jak6wrote:

I noticed something very strange with an Xperia Z5 i have.  I have two E6653's side by side flashed with the same firmware. One of them will only get 0-2 bars max on 3g+ the other easily has full bars on 3g+.  The one with 0-2 bars on 3g+ takes extremely long to load pages etc, and sometimes it will not load at all even with 2 bars. The APN settings are exactly the same on each phone and SIM card is the same.  So there must be something wrong with the hardware. So i replaced the antena and even the battery in the problematic Z5 but the results are the same.  Im suspecting something on the mainboard itself is to blame becasue i dont think there are anymore parts to replace that are responsible for signal strength. 

Does anyone know if there are any other parts on the mainboard that effect the signal that could be replaced / soldered?

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The SIM card is the same, what do you mean by this?

Because I asked my Telco for a new SIM, because replacement is free and when I changed it, the Signal Strength was much better

[NunoSEN2]

Hello @Jak6;

I agree with @Kycpokemon, have you tried switching the SIM cards in both phones to see if the symptoms are the same?

To discard a software problem have you performed a software repair with Xperia Companion?

[Jak6]

By the SIM card is the same i mean it is the same SIM card im using. I remove one SIM card from the phone and install in the other. It works perfectly in one Z5 while it has very weak signal in the other Z5. I tested it on an Xperia XZ also and that same SIM card gets very strong signal on it.  

Yes i ran the repair with Xperia Companian but it didnt change anything. Both Z5's are running freshly flashed .ftf files of 7.1.1 32.4.A.1.54.  Im going to try downgrading the problematic Z5 to 7.0 and see if that helps, maybe the mainboard is a different revision or something and doesnt get along with 7.1.1.

[Isabella1]

I have the same problem. Worst of all is that most of the time it runs out of network, and when it picks up, it gets a few bars and it's a sacrifice to make the 3G / 4G work. Were you able to solve your problem?

[Jak6]

I downgraded the firmware back to version 7.0 and as far back as 5.1.1 but none of it fixed the issue. There is something just wrong with the mainboard of the phone and its more cost effective to just replace the phone instead of finding a replacement mainboard.