iPhone "No Service / Searching": SIM, Software, or Logic Board?

Before touching the board, rule out:

• Try another SIM, and try this SIM in another phone — is the problem the line or the device?
• Carrier/coverage: do other phones get signal in the same spot?
• Reset network settings, check for updates; if needed, do a restore. A restore rules out software-side modem lock-ups.
• The device's registration/lock status (carrier lock, blacklist) can be queried — that's not a hardware fault, but it produces the same symptom.

These steps solve a serious share of cellular complaints without picking up a screwdriver.

Check the IMEI under Settings → General → About (or by dialing *#06#):

• IMEI shows but there's no service → the modem is alive; suspicion moves to the antenna/RF chain or the line/registration side.
• IMEI blank/unreadable → the modem area never introduces itself to the system; this picture points strongly at the logic board.

This single check is one of the most valuable clues, splitting the diagnosis into two separate paths.

If the IMEI reads but signal is weak or absent, look at the mechanical side of the RF chain:

• Antenna flex cables and coaxial connectors may be unseated, crushed or oxidized — reassembly mistakes are a classic on previously opened devices.
• Chassis deformation (drop/bend) can ruin antenna contact points.
• On devices with liquid history, oxide on the connectors seriously weakens the signal.

Microscope inspection of the connectors plus properly reseating the antennas resolves many cases in this family.

If the IMEI won't read, or a restore aborts with a specific error, the modem area on the board is next. This area has its own supply rails and data lines talking to the processor:

• Modem supplies are measured — a missing rail means the area never wakes up.
• Diode mode hunts for shorts/breaks on the lines; liquid and impact love leaving traces here.
• Modem-area faults sit among the advanced cases: diagnosis demands schematic/boardview reading and disciplined, systematic measurement.

Important note: some ICs in this area are paired to the device; uninformed part swapping can leave the phone worse off. That's why diagnosis is worth more than intervention.

Two extreme mistakes show up in cellular faults: calling everything "software" and spending weeks on restores, or the opposite — stamping it "modem failure" immediately and diving into heavy intervention. Both are diagnosis-free work.

The systematic order (SIM/software → IMEI check → antenna/connectors → modem-area measurement) shrinks the suspect list at every step; intervention comes only after the evidence is in.