Hidden iPhone code reveals your true signal strength

Most people only glance at the bars in the top right of the screen to judge how good their iPhone’s reception is. Apple, however, has a hidden testing mode built in that shows what is really going on, down to the exact dBm.

What is Field Test Mode

Field Test Mode is an engineering feature Apple originally intended for network testing, but it is accessible on every iPhone. Instead of the familiar bars, this mode shows the exact signal strength, the cell tower the device is connected to, the LTE or 5G band in use, and a list of other nearby towers.

How to open Field Test Mode

Open the Phone app and dial *3001#12345#*, then press the call button. This has to go through Apple’s own Phone app: an alternative dialer or VoIP app will not open this feature. No actual call is placed, instead the Field Test interface opens directly.

Reading the signal strength in dBm

Inside Field Test Mode, under the LTE tab, you will find the values rsrp0 and rsrp1: this is the signal strength in dBm, or decibel-milliwatts. The closer this number is to zero, the stronger the signal. A value between -50 and -70 dBm is excellent to good, with smooth calls and fast data. Between -70 and -85 dBm is still usable but may cause occasional hiccups. Below -90 dBm things get noticeably weaker, and below -100 dBm there is often barely any usable signal left.

What this is useful for

This feature is mainly useful for tracking down the cause of poor reception, dropped calls, or slow mobile data. By checking the dBm value in different spots around the house or office, it quickly becomes clear where the signal is weakest, for example in a basement or behind thick walls. It also shows whether an iPhone has switched to a weaker band or a more distant cell tower, which can explain why speed suddenly drops.

Anyone dealing with inconsistent reception on a regular basis can use this code as a kind of built-in signal meter, without needing a separate app for it.