What you'll see

Both units are running. The blower is moving air, the condenser fan is spinning, the compressor is humming along — and the customer is still hot. Supply air feels cool-ish but not cold, or the system runs nonstop and never satisfies the thermostat. This is the "it works but it doesn't" call, and it almost always lands on the refrigerant/airflow side rather than the electrical side, because everything is energized and turning.

Your goal is to figure out why heat isn't moving, and you do it by reading the system as a state — not by chasing one gauge needle.

Walk it in order

  1. Confirm the complaint and the conditions. Put your hand on the supply air, then the return. Measure the split with a real thermometer, not your palm. Note outdoor temp and indoor humidity — a system fighting a 100°F afternoon with the doors propped open isn't broken. Ask what changed: did it die slowly over weeks (leak) or quit suddenly (component)?
  1. Rule out airflow before you touch the charge. This is the step techs skip and regret. Pull the filter — if it's gray and packed, that's your lead. Check the blower wheel for a coating of dust (a clean filter in front of a caked wheel still starves the coil). Look at the evaporator for dirt or partial ice. Confirm the blower is actually on the right speed for the tonnage. A starved coil mimics low charge perfectly on the suction gauge.
  1. Check the condenser side outdoors. Dirty coil, cottonwood mat, a fan that's running slow, or recirculating hot air against a wall all raise head pressure and kill capacity. Feel the air coming off the condenser — it should be hot. Lukewarm discharge air means it's not rejecting heat.
  1. Now put the gauges on and read the whole picture. Suction, head, superheat, subcooling, and the indoor split — together. One number lies; the set tells the truth.
  1. Interpret the state:
  • Low suction + high superheat + low-ish head + low subcool → undercharge or a leak. Look for oil stains, weigh the charge, find the leak.
  • Low suction + high superheat + normal/high subcool → restriction (clogged drier, stuck TXV, kinked liquid line). The subcool stacking up is the tell. Feel for a temperature drop across the drier.
  • High head + high subcool → overcharge, or a condenser that can't reject heat (dirty coil, slow/dead fan, recirculation). Clean and confirm fan amps before condemning charge.
  • Near-normal pressures + weak split → airflow problem you missed, or the equipment is simply undersized/overloaded for the load (leaky duct in a hot attic, massive solar gain).

What "normal" looks like (R-410A, moderate day)

  • Supply-to-return split: about 16–22°F across the indoor coil at normal humidity. A 6–10°F split screams airflow or low capacity. A 25°F+ split usually means low airflow over the coil.
  • Suction: roughly 115–135 psig (sat ~38–45°F).
  • Head: roughly 350–425 psig; condensing temp ~15–25°F over outdoor ambient.
  • Superheat: TXV ~8–12°F; a piston system varies with conditions — high superheat anywhere points to a starved coil.
  • Subcooling: commonly ~8–12°F on a TXV system. Manufacturer's spec wins over rules of thumb.

These shift with weather and humidity. Use them to spot what's abnormal, not as pass/fail lines.

Common faults & what they mean

  • Low charge from a leak: the slow-fade complaint. Long run times, low suction, high superheat, sometimes sweating or ice on the suction line. Find and fix the leak — don't top off.
  • Dirty/blocked condenser or weak condenser fan: high head, high subcool, marginal cooling that's worst on the hottest days.
  • Restriction: high superheat with low suction, but subcool that's normal or high. Temperature split across the drier confirms it.
  • Airflow starvation: dirty filter, plugged coil, caked blower wheel, crushed return — low suction with normal/low superheat, high split.
  • Undersized/overloaded: everything reads near-normal, split is okay at the coil, but the structure gains heat faster than the unit removes it. Duct losses in a hot attic are a huge hidden cause.

Tech tips & gotchas

  • Clear airflow first, every time. More "low on refrigerant" calls are actually dirty filters and plugged coils than anyone wants to admit. Adding gas to an airflow problem makes it worse and can freeze the coil.
  • If the coil is iced, you cannot read charge. Thaw it fully (fan-only), fix the airflow or charge cause, then read.
  • Discharge line temperature is a free clue. A scroll running with very high discharge temp often points to high superheat / low charge starving it.
  • Check the duct, not just the equipment. A perfect 18°F split at the coil with 80°F air at the far register means the cold is leaking into the attic, not into the house.
  • Two readings make a data point; five make a diagnosis. Commit to writing down suction, head, superheat, subcool, and split before you decide.

Safety / code notes

  • De-energize and verify before pulling the condenser panel; bleed the run capacitor — a "dead" cap can still bite.
  • Recover and handle refrigerant per EPA 608 (40 CFR Part 82, Subpart F) any time you break into the sealed system.
  • Wear eye protection on the gauges — liquid R-410A can flash-freeze skin and eyes.