How-To Geek
How to Access Your iPhone’s Field Test Mode (and See Your Real Signal Strength)
The iPhone has a hidden “field test” mode that shows all kinds of technical details about signal strength, cell towers, and more. Most of it is not very useful to the average person, but you can make it show you your phone’s actual signal strength instead of just how many bars you’re getting. And that can be useful.
Even in urban areas dense with cell towers, signal strength can vary widely depending on the phone’s carrier and current location. Things get worse in rural locations, where large areas may be covered only by a single carrier’s tower, or in a house with thick walls your signal won’t penetrate (in which case a signal booster can help).
In any of these situations, knowing your phone’s precise signal strength rather than just a vague range of 1-5 bars can really help you diagnose the problem, and figure out the best way to fix it. And that’s where your iPhone’s field test mode comes in.
You can access the field test mode on any iPhone. All you have to do is fire up your phone app, dial the following code, and then tap the Call button.
*3001#12345#*
Your iPhone will enter a field test mode that offers up several menus of technical measurements. Most of these are only useful if you’re developing phones or testing cell towers.
What you’re after, though, is not on the menu. If you’re using iOS 8 or any earlier version, your exact signal strength is displayed in the upper left corner of the field test screen. If you’re using iOS 9 or later, that reading is replaced with the “Back to Phone” link instead.
To view your signal strength in iOS 9, you’ll use a little trick to replace your bars on your main screen with the signal strength measurement (and you can also do this on earlier versions of iOS if you want). Hold down the Power button until the “Slide to Power Off” message appears, but don’t power off. Let go of the Power button and then press and hold your Home button until your home screen reappears. You should now see the signal strength displayed where your bars used to be.
This change will last until you restart your phone (or until you repeat the process above). You can also tap the signal strength number to switch between signal strength and bars.
Once you have your signal strength visible, you can walk around your home (or whatever location you’re testing) and see the precise signal strength wherever you go. Just keep in mind that strength is shown in decibels and will be on a negative scale (so, a signal of -75 is stronger than -115). If you notice that your signal strength is weaker inside your home, but reasonably good outside, you may be a perfect candidate for a signal booster. You can even walk the outside of your home to find where the signal strength is strongest, so you know the best spot to place an external antenna.
Exact signal strengths will vary a bit depending on your carrier and whether you’re on a 3G or 4G network. In general, though, you can consider a signal above -80 to be close to full strength (full bars) and a signal below -110 to be very weak (one bar).
While there’s a lot more in the field test mode that might be of interest to developers and cellular techs, being able to see the actual signal strength rather than just how many bars you have can really help you determine the optimal placement for a signal booster. It can also help you test service in certain areas (like your home) on your friend’s phones before you commit to a carrier.

Also, you can toggle between dB and bars by tapping on the icon.
I love these kinds of things. I need a service menu like this for WiFi....
I agree with wilsontp! Need an RSSI in dBm for WiFi sigs... Gotta be a way of doing that too.
Okay, I'm at a loss. I followed the instructions, got to the field test, tapped back to iPhone and the number flashed briefly, but didn't stay, just back to the bars immediately. Tapping the bars does nothing. Is this something only available to iPhones in the States? I also followed the instructions to have it stay on my home screen, but they didn't work.
Hold down the Power button until the “Slide to Power Off” message appears, but don’t power off. Instead, press and hold your Home button until your home screen reappears. You should now see the signal strength displayed where your bars used to be.
It doesn't state to continue holding the Power button when you press and hold the Home button so I did this twice, once letting it go and then pressing the Home and then doing it holding it down. Still just bars appear on my Home.
@LadyWrites If you tap back to phone nothing will change.
While you're looking at the field test mode screen, press and hold the power button until the "Slide to Power Off" message appears and then let go of the power button. Then, press and hold the Home button until you are returned to your home screen. Then you can let the home button go.
That should change you to showing the signal strength instead of bars.
Level reading stays on for about 1 second then reverts to the "dumb bars". Power/home button does nadda. Is there a list of secret squirrel codes for other cool stuff? The stuff in the menu is neat-o!
I ended up buying a cheap Android phone to get that info.![:frowning: :frowning:]()
@robb after you do it can you then tap the bars to switch it back and forthbetween signal and bars?
I've tried this on several iPhone models so I'm kind of curious why it'snot working for you.
Thats a damned good idea! I will look for one. Have you tried the applicable apps for WiFi sig analysis? Which one do you like?Thank you!
WalterNo joy. Tried with WiFi & LTE disabled along with other stuff. Level reading comes up for about one second then reverts to dumb bars. Tapping on the dumb bars changes nothing. Using 9.3.1. Anything else you can think of?Thank you!
@robb if you pull down your Notification Center does it show up? I can'tthink of any reason it wouldn't work for you. I don't have the SE to try itout on but it works on my 6s with 9.3.1.
It's also been around since at least iOS 8.
I tried this, and it worked well. I cannot get the phone to revert back to the bars for good. I rebooted the phone (powered off then on) and I also tried a hard boot (hold power and the home button) but I can still swap between the bars and the numbers.I just want to put the phone back like it was. Is there a downside to leaving it like this?
Ok, I figured it out. Just go back into Field Test Mode, then press Home button to exit. No worries, I got this. lol