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Repair information and guides for the iPhone 6S released by Apple on September 25, 2015. Models: A1688, A1633

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Service Provider in settings keeps appearing and dissapearing

This phone has been previously liquid damaged and cleaned very well on the logic board.

Now the leftover issue is:

Service Provider option in settings keeps appearing and dissapearing, phone sim card detect fine, goes searching then no service.

IMEI and modem firmware displayed in settings, checked *#06* no problem there too.

Is it okay to reflow the U_WTR_RF transceiver chip because of liquid damage before? Maybe it'll fix the problem?

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If the IMEI is showing, then the problem is not the usual long-jumper Baseband repair. You could try to reflow U_WTR_RF but I would probably reball it instead. You could also look at U_ASM_RF & U_WFR_RF. @teetopp summarizes it well in this answer (Service is "Searching" indefinitely).

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