Field Test Mode is a hidden feature on the iPhone which allows you to see technical details of the device, the most useful of which is the true signal strength displayed as a number rather than the traditional signal bars.
Entering Field Test Mode on iPhone
This will work on any iPhone model except the original:
- From the iPhone keypad, dial *3001#12345#* and hit “Call”
You’ll immediately see the signal numbers in the upper left corner, and you can tap around the menus to discover other random features and information that is generally meaningless outside of cell technicians and field operators. If you hit the Home button you’ll quit out of Field Test and the signal indicator will return to bars rather than the signal numbers, but it’s easy to always see the numbers too as described below.
Enabling Signal Number as Reception Indicator Rather Than Signal Bars
To always see the signal numbers rather than the signal bars, you’ll use the Force Quit app function to kill Field Test when it’s open:
- Dial *3001#12345#* and hit “Call” if you haven’t done so already to launch Field Test
- Now hold down the Power button until the “Slide to Power Off” message appears, then release the Power button and hold the Home button until Field Test quits
- Tap the signal bars or signal numbers to switch between the two
To remove the tap-to-switch signal indicator ability, you can either reboot the iPhone or go back into Field Test and close out of it as usual.
How to Read the Field Test Signal Indicator Numbers
The numbers don’t follow a scale that makes much sense to normal people, but the lower the number (in other words, the more negative) the worse the signal, and the higher the number (less negative) the better.
- Anything above -80 is good, and would be considered full bars
- Anything below -100 is bad, and would be considered few bars
For example, a signal number of -105 is considerably worse than a signal of -70. You’ll generally find that anything approaching -100 or lower is fairly bad reception, while anything above -80 is usually good, and if you tap the number signal it’s usually shown as full bars. The full range of the signal numbers extends from -40 to -120, with -120 being a nearly impossible number to see because it means no reception, and -40 would be about the strength you’d get being right alongside a cell tower.
This is actually a fairly old hidden feature that works on any iPhone running iOS 4.1 or later, but we’ve had a lot of questions about it recently due to several recent tip screenshots showing the signal numbers.
Extracted from osxdaily.com