Save time using a test cable
When it comes to screen replacement the iPhone 4 and 4S are arguably the most complicated phones that Apple has ever designed. The iPhone 2G was pretty tough to repair also but since you are not likely to come across one worth fixing we will ignore that model.
Because of the significant time investment required to perform this repair it is a good idea to be certain that your replacement part is functioning properly before you take on the time consuming process of gutting the phone in order to remove the broken screen. Fortunately the display interface if accessible with the removal of just a few pieces from the back so you can plug in your new part and check it for any problems before performing a full installation. This is made possible with the use of a test cable that will only cost you a few dollars. What’s great about these test cables is that not only are they inexpensive and easy to use but the 4 and 4S use the same cable so there is no need to purchase more than one. Regardless of your carrier they work the same way so even CDMA phones from Verizon or Sprint are supported.
The test cable can be purchased through the link below and if you scroll down the page a bit you will see a video tutorial demonstrating how they work.
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You will have to remove the back panel from the phone as well as the EMI shield located at the upper end of the logic board. Be sure to disconnect the battery before you remove any screws or start unplugging cables. It is always best to make sure that there is no power running through the phones circuitry when you connect or disconnect any of its components. Also keep in mind that if you decide to perform a test call while this cable is attached to an external LCD you will have to find a way to get the existing screen to face a light source. Because the proximity sensor is located on the front side of the phone it will shut the screen off during a call if you have it covered by the surface that you are working on.
Video: How to use a test cable
Besides looking for bad pixels we recommend testing the touchscreen for dead spots while you have the test cable connected. This is especially important in the areas where text and email composition take place. The best way to do this is to click every letter and symbol on the screen to be sure that they register. Keep an eye out for any duplicates as well since this would be an indication of a defective digitizer.
Most suppliers are very good about returning or exchanging a display assembly that has problems before or after installation assuming that it is still within the warranty period but this is best avoided by checking for problems as soon as your part arrives. If there is something wrong with it you don’t want to go through all of the steps required to remove it and if you were to damage it during the process you warranty will be voided. While we rarely see defective parts from the sellers that we use it’s better to be safe than sorry.
Good luck with your iPhone 4 repair. We hope that using a test cable saves you some time.