Symptom ID — Confirming the Head Is Restricted, Not the Whole Zone

Low flow at a sprinkler head looks similar to a zone-wide pressure problem, so localize before touching anything. Walk the zone while it runs and look for one of three patterns. (1) Single head sprays weakly while all other heads in the zone are normal. This is the classic clogged-head signature — debris, scale, or a torn filter screen on one body is starving that single head while the rest of the zone runs at full pressure. (2) A small cluster of heads (2–4) on one lateral are weak, while heads on the next lateral run normally. This points to a partial blockage between the lateral tee and the cluster — a clog or a marginal compression fitting ahead of the affected group. (3) Every head in the zone has reduced output, but the pressure gauge at the next-downstream head reads normal. This is a zone-wide pressure issue, NOT a head clog — proceed to the manifold/valve diagnosis instead. The no-fault baseline: a residential spray head at 30 PSI throws 12–15 feet; a residential rotor at 30 PSI throws 20–30 feet. Anything meaningfully short of the rated radius, on a single head, while neighbors are normal, is a head-local restriction.

Debris and Clog Inspection — Nozzle Orifice and Filter Screen

Run the affected zone briefly and watch the affected head while it sprays. Note the spray pattern — uneven, fan-shaped, or reduced radius all suggest a partial orifice blockage rather than a fully blocked head.
Shut off the zone at the controller. Pull the riser up by hand and lock it in the raised position (most pop-up risers have a small pull-tab or a 1/4-turn lock near the cap).
Unscrew the nozzle counter-clockwise with a small flathead screwdriver or by hand (depends on the model). Set the nozzle face-down on a clean shop towel — do not set it on dirt or grass, where the orifice can pick up debris that you'll then install back into the head.
Inspect the nozzle orifice against a bright sky or flashlight. A healthy nozzle has a clean, sharp-edged circular or arc-shaped opening. A clogged nozzle shows mineral scale (chalky-white ring around the edge), sand grains visibly lodged in the opening, or organic debris (root fragment, insect, spider web) across the path.
Remove the cap and riser seal sub-assembly by lifting straight up out of the body. Set these parts on the towel. The riser seal at the base is a small rubber or plastic ring — inspect for tears or hardening; a torn seal lets water bypass the pop-up mechanism and reduces flow.
Locate the filter screen at the BASE of the riser column, where water enters the body. The screen is a small cylindrical mesh — pull it out with needle-nose pliers.
Inspect the screen against the light. A clean screen is uniform fine mesh; a clogged screen shows visible fouling (sand, scale, organic matter) and may have visible tears or holes if the mesh has been damaged during a previous service.
Rinse the nozzle and the screen under clean running water. Use a soft brush (an old toothbrush works) to scrub mineral scale from the nozzle face — do NOT use any metal tool, which will enlarge the orifice and permanently change the head's GPM output.
If fouling is heavy, soak the nozzle and screen in white vinegar for 10 minutes, then re-rinse. Vinegar dissolves mineral scale without damaging plastic.
Re-install the screen, the riser seal sub-assembly, and the nozzle. Run the zone; confirm output is restored. If output is still restricted after a clean screen and clean nozzle, the restriction is upstream of the head — proceed to the pressure test.

Static vs. Dynamic Pressure Test at the Affected Head

A clogged head produces a specific signature on a pressure gauge — high static, low dynamic, and a wide gap between the two. Use a hose-thread pressure gauge that screws onto a hose bib or directly onto a head riser removal socket (a Pitot gauge is more precise, but a hose-thread gauge is sufficient for residential diagnostics). Step 1 — Static pressure. With the zone off and all other zones off, screw the gauge onto the nearest hose bib or a head riser with a gauge adapter. Static pressure on a residential irrigation system should read 40–80 PSI. Below 30 PSI static is a supply-side problem (regulator set wrong, marginal city supply, partially closed master valve) — no amount of head cleaning will restore output. Step 2 — Dynamic pressure at the affected head. Open ONLY the affected zone. Read the pressure at the same gauge location. A healthy rotor zone reads 30–50 PSI dynamic. A healthy spray zone reads 25–35 PSI dynamic. A drop of more than 20 PSI from static to dynamic indicates a restriction that is NOT the nozzle itself — suspect a clogged master filter, a marginal valve, or a pinched lateral pipe. Step 3 — Dynamic pressure at the affected head directly. Move the gauge to the affected head (use a Pitot gauge on the spray stream, or a gauge adapter screwed onto the spray head body). The pressure at the head should be within 5 PSI of the manifold reading. A drop of more than 10 PSI between the manifold and the head indicates a restriction between the two — most often a clogged master filter, a marginal solenoid port on the zone valve, or a partially closed valve flow-control stem.

Filter-Screen Health Across the Zone

If one head has a fouled screen, neighbors usually have fouled screens too. A salt-water mulch layer of mineral scale means the supply-side water has high hardness — every head will accumulate scale over the season. A sand layer means the supply is carrying sediment — usually from a recent main-line work, a torn master filter, or a degraded well. Walk the entire zone and pull a screen from 3–4 heads at random. If they are all fouled to a similar degree, the failure is supply-side (the symptom is showing up first at one head but the same fouling will reach every head within the season). Schedule a master-side filter inspection and a full zone flush — pull every head's screen, rinse, and reinstall. If only one screen is fouled, the source is local debris — a piece of gravel fell into the body during a recent installation or repair. Replace just that screen.

Nozzle-vs-Head Failure Triage

Low flow can be a nozzle problem, a head-body problem, or both. Triage in this order: (1) Nozzle clog (most common). Spray pattern is uneven or reduced, but the head still pops up and retracts. Fix by cleaning or replacing the nozzle. (2) Filter-screen clog. Same symptom as nozzle clog, but the nozzle itself is clean when you inspect it. Fix by cleaning or replacing the screen. (3) Cracked riser. The head pops up partially, then sinks mid-cycle; water visibly leaks at the riser-to-body joint while the zone runs. Water bypassing a cracked riser drops the differential pressure needed for pop-up, so the head acts like it has low flow. Replace the entire head — no internal repair will seal a cracked riser. (4) Weak return spring. The head pops up weakly at low pressure but will not reach full height when pressure is good. Replace the head — the spring seat is part of the body and is not serviceable as a separate part on most residential heads. (5) Undersized nozzle GPM. The nozzle is clean and the head is mechanically sound, but the spray radius is still short. Confirm by reading the nozzle label stamped on the cap or body — if a smaller-radius nozzle was installed by mistake (e.g. a 10-foot nozzle on a 15-foot head), replace with the correctly-rated nozzle for that head model. (6) Internal damage from freeze. The head shows no nozzle clog, no screen clog, no cracks visible from outside, but the spray pattern is wildly inconsistent. The internal wiper seal and riser guide may be deformed from a past freeze event. Replace the entire head.

Root-Cause Summary

Clogged nozzle orifice — most common; clean with a toothbrush, never metal tools.
Clogged filter screen at the riser base — clean or replace; check neighboring heads for the same fouling pattern.
Supply-side sediment — replace the master filter; flush the zone end-to-end.
Cracked riser or split wiper seal — head replacement only; no internal repair will seal a cracked riser.
Weak return spring — head replacement only; spring seat is part of the body on most residential heads.
Undersized nozzle (installed wrong radius) — replace with the correctly-rated OEM nozzle.
Freeze damage to internal wiper seal or guide bushing — head replacement only.
Low static pressure (zone-wide, not head-local) — investigate the regulator, master valve, and supply shutoffs.

Repair vs. Replace Decision for the Affected Head

A cleanable nozzle, a replaceable filter screen, and a swappable riser seal are all single-component repairs on a still-functional head body. Replace the entire head when the body is cracked, the riser is bent, the spring seat is visibly corroded from mineral buildup, or the head is more than 10 years old and parts are no longer stocked. Premium residential heads (Rain Bird 1800 series, Hunter PGP/Pro-Spray, Toro 570Z) are worth rebuilding — nozzle + screen + seal cost $5–10 versus $15–25 for the whole head. Commodity heads from big-box stores are usually not worth internal repairs — the bodies are lower-grade plastic and seal fitment is inconsistent between production runs. Default to replacement on commodity heads, default to rebuild on premium heads.

Cross-Reference to the Repair Guide

Once you have localized the failure (cleanable nozzle, replaceable screen, cracked riser, weak spring, undersized nozzle, or supply-side sediment), proceed to the steps in the repair library. For the full walk-through — nozzle removal/cleaning, filter-screen replacement, riser replacement, post-repair runtime check, and GPM-match re-nozzle guidance — read the Sprinkler Head Clog Repair Guide in the Repair Library. The Blueprint Bundle below includes both this diagnostic guide and the matching repair guide so you have everything you need to go from symptom to verified fix.