It is not uncommon for a lawn sprinkler head to leak for a number of different reasons. Fortunately, repairs are relatively easy to conduct using a few simple steps. As pop-up sprinkler heads are the most common, the below steps will assist in resolving leaks common to that type.
Step 1 - Conduct a Visual Inspection
The first step to fixing the leak is determining where it is. To do this, turn on the sprinkler, and pay attention to where the water is coming out. Leaks at different areas can be attributed to different causes. Check to see if the water is leaking where it sprays out the head or if it is above or below the head itself. Water can also escape out of the sprinkler housing or from the ground below.
Step 2 - Check the Sprinkle Head Assembly
If water is leaking out of the head of the sprinkler, it may be loose, clogged, or damaged. It will be easier to check the head for issues if you remove it, so use the shovel to dig it up. Most sprinkler heads are removed by twisting a cap off the top of the sprinkler body or housing. The cap typically holds the shaft that pops up when water pressure is added. This cap also keeps water from leaking out of the body. Inspect the seal at the bottom, and then push up on the shaft to expose the sprinkler head itself.
Most heads unscrew from the shaft. Once you've removed it, turn the shaft upside down and shake it. This should dislodge the screen or filter inside. If not, use a long flathead screwdriver to push the filter out from the bottom up. Inspect the filter for any debris. If debris is present, flush the filter with water.
Now inspect the sprinkler head you removed. See if any debris is lodged inside the spray area of the head. Also look at the threads on the bottom of the sprinkler housing for any damage. If there is debris or damage present, the sprinkler head will need to be replaced. If not, continue your inspection.
Step 3 - Inspect the Riser and Fitting
If water was visible from below the sprinkler body, the leak may be at the riser or fitting. Use the shovel to dig down to the fitting, and then turn on the water. See if any leaks appear at the fitting. If so, you will need to replace it.
In the event that the water leaks where the riser connects to the fitting, you'll want to remove the riser. Inspect the threads on both ends for any damage. Go ahead and reattach it if the threads check out, and turn the water back on. If the leak persists, you can wrap Teflon tape around the threads two or three times before you test again.
As a last resort, you can look into replacing the leaking sprinkler with a new one. This can be trying for a DIYer, so be sure that's necessary before doing so.