You don't have to test all fuel injectors. For example, you might have Cylinder #4 Misfiring or dead and you want to just test that particular fuel injector... well that's OK.
My suggestion is to test the two adjacent fuel injectors so that you can have at least two other resistance (Ohms) value to compare the one you suspect as BAD.
If you don't have a specific misfire code to go on... well that's OK too... just test all of the fuel injectors.
Now, let's say that the one you want to test is under the intake manifold plenum and you need to remove it to get to it... well, in this situation you can also only test the one you think is BAD and at least two others, instead of all of them.
Diagnosing a BAD fuel injector on your 4.0L Ford can be a challenge... since you need to remove the intake manifold's plenum to test 3 of them. So, in this section, I'm gonna' offer you my fuel injector troubleshooting strategy so that you can get to the bottom of the problem without wasting time and money.
These are the steps I take:
Remember, the most important step... is to find the ‘dead’ cylinder first! Finding the misfiring cylinder is half the battle to correctly diagnosing the engine miss (misfire, ‘dead’ cylinder, etc.) that's afflicting your 4.0L Ford Explorer (Ranger, Aerostar or Mountaineer)!
Here are links to other articles that may be of help in troubleshooting your Ford 4.0L equipped vehicle that I've written. To see the articles in this Web Site, click here: Ford 4.0L Index of Articles.
Here's a sample of the tutorials you'll find in the index:
“For instance, on the planet Earth, man had always assumed that he was more
intelligent than dolphins because he had achieved so much —the wheel,
New York, wars and so on —whilst all the dolphins had ever done was
muck about in the water having a good time. But conversely,
the dolphins had always believed that they were far more
intelligent than man —for precisely the same reasons!”
The Hitchhiker's Guide to the Galaxy