Field Test Mode is a hidden feature on your iPhone which allows you to view useful information about your phone including the true signal strength. It will transform signal bars to show the actual signal in decibels. You may want to use this feature because signal bars may not be very accurate, while decibels are very precise. Do not worry, no jailbreaking is required and it’s easily reversible. Here is how:
1. Open the Phone app, switch to the keypad and dial *3001#12345#* and then hit call.
2. You’ll now enter Field Test mode. You will also see the signal numbers in the upper left corner. You will see a lot of other information that is usually meaningless outside of phone or cell technicians.
3. Press the home button to exit Field Test Mode or if you would like the feature to be permanent so that you will always see the numbers instead of the bars, continue on.
4. Hold down on the Sleep/Wake (or power) button until you see the ‘slide to power off’ bar, then release the Sleep/Wake button. Do not power off the device. Then ahold the Home button until Field Test quits and you return to your home screen. If you want to reverse this, dial *3001#12345#* on your iPhone then press Call then hit the Home Button once and and the signal indicator will return to bars.
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Field Test Signal Indicator Numbers
As you can see below, the lower the number (more negative) the worse the signal, and the higher the number (less negative) the stronger your signal.