The standard cellular signal bars displayed at the top right of your iPhone screen are notoriously inaccurate. “Four bars” of signal does not actually tell you how strong your connection is; it is simply Apple’s visual estimation. If you are experiencing dropped calls or terribly slow 5G data speeds, you can bypass the visual bars entirely and use a hidden diagnostic tool called “Field Test Mode” to see your exact, mathematical signal strength in true decibels.
How to Enter Field Test Mode
Field Test Mode is a hidden diagnostic app built deep into iOS. It has no app icon and cannot be found via Siri. The only way to launch it is by dialing a secret code into the standard Phone app.
- Turn off your iPhone’s Wi-Fi so the device is strictly using the cellular network.
- Open the standard Phone app and tap on the Keypad tab at the bottom.
- Carefully type the following exact code, including the asterisks and the pound symbols:
*3001#12345#*
- Tap the green Call button.
Reading the Diagnostic Data
Instead of making a phone call, your iPhone screen will flash and instantly launch the hidden Field Test diagnostic dashboard. This screen looks highly technical, but you only need to find one specific number.
- On the main dashboard, look for a menu item labeled LTE or 5G (depending on your current connection).
- Tap on Serving Cell Meas (Measurements).
- Look for a metric labeled rsrp0 or RSRP (Reference Signal Received Power).
This number is your true, mathematical cellular signal strength, measured in decibel-milliwatts (dBm). It will always be a negative number.
- A reading between -40 and -80 is absolutely excellent. You have a flawless connection.
- A reading between -80 and -100 is average to good.
- A reading between -100 and -120 is very poor. You will likely experience dropped calls and slow data.
You can walk around your house while watching this number update in real-time, allowing you to find the absolute strongest cellular signal in the building. When you are finished, simply swipe up to close the app normally.