After charging: runs a while and shuts off, restarts and shuts off again (cycling)

Tools: Gauges, clamp thermometer on suction and liquid lines, air thermometer (supply/return), amp clamp, unit data plate, SH/SC and Expected tabs

1. Conditions: outdoor 65 °F and up, indoor 70–80 °F, unit running 15–20 minutes, clean filter, indoor coil not plugged, condenser fan running. Without that any reading lies — airflow first, refrigerant second.
2. Sign 1 — SUBCOOLING (primary with a TXV/EEV; the plate or manual says “charge by subcooling 10–12 °F”). Measure liquid-line temperature at the outdoor service port and high-side pressure → SH/SC tab. Normal 8–14 °F (plate ±3). Subcooling 0–5 = TOO LITTLE (not enough liquid in the condenser). Subcooling 18–30 = TOO MUCH (liquid stacking in the condenser).
3. Sign 2 — SUPERHEAT (primary for cap tube/piston; a check for TXV). Suction-line temperature at the service port minus low-side saturation. Fixed orifice: target from the chart on the Expected tab (depends on outdoor temp and indoor wet-bulb), usually 10–25 °F. TXV: 8-14 °F at the condenser within a normal charge range. SH 25–40 = TOO LITTLE (starved evaporator, warm line). SH 0–3 with an icy line down to the compressor = TOO MUCH or TXV stuck open.
4. Sign 3 — SUCTION PRESSURE vs Expected tab. R-410A in summer: suction about 105–130 psig (evaporating 35–45 °F). Below 90–100 with a clean filter = low. Above 150 with high head = too much (or a weak compressor).
5. Sign 4 — AIR ΔT and frost. Supply−return: target 16–22 °F at normal humidity. ΔT 8–12 + frost on the suction line/service port/evaporator = low refrigerant (evaporating below 32 °F). ΔT normal but head high and liquid line hot = too much.
6. Sign 5 — COMPRESSOR AMPS. RLA on the plate is a rating figure for sizing wire and breakers, not a “normal”; expected amps depend on the pressures (compressor maker's data). Low charge → amps below expected, compressor hot to the touch (little gas to cool it). Too much → amps above expected, high head. Amps above RLA are always bad.
7. Summary: low = low SC + high SH + low suction + frost + low amps. Too much = high SC + SH near zero + both pressures high + high amps. Signs pointing different ways (high SC, high SH, low suction) = restriction, not charge — see the restriction/TXV symptom.
8. Charging rule: TXV — by subcooling in 4–8 oz steps, wait 5–10 minutes between. Fixed orifice — by superheat. Never “by pressure by eye” and never “until the line sweats”.
9. In doubt — recover everything and charge BY WEIGHT: plate (oz) + line length (Top-off tab → weigh-in). That's the only way to know for sure.

Tools: Gauges, two clamp thermometers, air thermometer, the SH/SC, Expected and Standing tabs

1. Logic. A liquid-line/TXV restriction raises head but CRASHES suction (starved evaporator, high superheat). If low and high rise together, there's too much of “something” in the system (refrigerant or air) or the condenser isn't rejecting heat. Then tell them apart by the numbers.
2. Number 1 — subcooling (SH/SC tab, liquid/bubble). 18–30 °F and up with high head = liquid stacking in the condenser = OVERCHARGE (or air, see number 4). Normal/low (0–10) with high head = the condenser isn't rejecting heat: dirt, fan, recirculation, heat wave.
3. Number 2 — superheat. Around 0–3 °F, suction line icy down to the compressor = overcharge (liquid to the compressor). Normal 8–14 = more likely condenser/air. High 20+ with high head and LOW suction = restriction, a different symptom.
4. Number 3 — condensing over ambient (CTOA): high-side saturation minus outdoor temperature. Normal 15–30 °F by SEER (Expected tab). 35–50 °F = the condenser can't keep up, or overcharge/air. Touch: liquid line too hot to hold, discharge at the compressor > 225 °F — poor heat rejection.
5. Number 4 — standing pressure (Standing tab): shut off, let it equalize 20–30 minutes, compare with the chart at the coldest place's temperature. 10+ psi ABOVE the chart on a cooled unit = air/nitrogen in the system. Matches = no air, so overcharge or condenser.
6. Quick test overcharge vs air: recover 4–6 oz into a cylinder while running. Overcharge — head and subcooling drop noticeably right away. Air — head barely changes, subcooling “floats”, standing pressure still above the chart. Never vent to the air.
7. Quick test for the condenser: wash the coil inside-out and check fan speed/amps, clearances around the unit. Head dropped 50–100 psi — that was it. A fan turning slowly / blowing the wrong way (after a motor swap) is a common cause of “700 psig”.
8. Don't confuse with load: 105 °F outside + 85 °F house + windows open give both pressures above normal with normal SH/SC. Run 20–30 minutes and compare with Expected for that outdoor temp.
9. 700 psig on R-410A: the high-pressure switch must cut the compressor at ~610–670 (exact value per the unit's manual). If it reached 700 — the switch is jumped, failed or missing. Find and restore it BEFORE any charging: this is a burst line and an injury.

Tools: Gauges, thermometer, electronic leak detector, bubble solution, UV dye (optional), recovery machine/scale

1. Picture: low suction (R-410A below 100 psig, evaporating below 32 °F), high superheat 25–40, subcooling 0–5, frost on the suction line/service port, evaporator icing from the bottom up, air ΔT 8–12, compressor hot, amps below RLA. Head is LOW for that outdoor temperature too: condensing only a few degrees above outdoor (CTOA below normal) — little refrigerant, little in the condenser too.
2. Don't confuse: same low side, but head NORMAL/high and subcooling HIGH = restriction (drier/TXV), not undercharge. Add refrigerant and you make it worse.
3. Don't confuse 2: low suction + LOW SH (cold line) = too little AIR across the evaporator (filter, coil, blower, closed registers) — not the charge either.
4. Check standing pressure (Standing tab): with the unit off and equalized, pressure 10+ psi below the chart for the coldest place — really low (or the wrong gas in the system).
5. FIND the leak before charging: detector on every braze joint, Schraders (under the caps), TXV, evaporator (through the access panel), condenser coil U-bends; bubbles on anything suspect; for a micro-leak — nitrogen 150 psi + bubbles, or UV dye and a check in 2–4 weeks.
6. Charging: TXV — by subcooling; fixed orifice — by superheat; after a big loss — recover and weigh in. Log how much was added (oz) and the date — the Top-off tab keeps history; rule of thumb: losing more than ~10 % of the charge per year — offer a leak repair or replacement (EPA leak-repair rule: from 2026, HFC systems with 15 lb or more: 10 % comfort cooling, 20 % commercial refrigeration; residential/light-commercial AC exempt; R-22 systems: 50 lb or more).

Tools: Gauges, multimeter, amp clamp, thermometer, eyes and a watch

1. Watch HOW it shuts off. Outdoor unit goes quiet while the indoor fan keeps blowing → an outdoor safety tripped (pressure or compressor overheat). Everything stops at once → thermostat, board, drain float, power. Compressor silent while the condenser fan spins → compressor thermal overload or contactor/compressor capacitor.
2. Time it. Shuts off 1–3 minutes after start with head climbing toward 600 psig → HIGH-pressure switch → overcharge, dirty condenser, fan, air in the system (see “head 500–700”). Shuts off after 5–15 minutes with suction falling to 20–50 psig → LOW-pressure switch → undercharge/leak, freezing evaporator, plugged filter.
3. Sits 5–30 minutes and restarts by itself with no reset → compressor thermal overload. Cause: liquid into the compressor (overcharge, SH≈0) — compressor icy/sweating; or too little refrigerant to cool it (SH 30+) — compressor scorching; or a weak compressor capacitor (high amps); or low voltage.
4. Do the charge check (procedure “Charge check: too much or too little”) — it answers the main question. Charging “by pressure” without subcooling is the most common cause: on a cool evening you filled to a “normal” 120 psig, and at noon that's an overcharge.
5. If it's low and you just charged — the leak wasn't found. Standing pressure and an electronic leak detector on every joint, the service ports (caps with seals!), Schraders, the evaporator. Charging a third time without a leak search is the customer's money down the drain.
6. If the charge is right and it still cycles — look at the electrical: compressor capacitor (µF vs plate ±6 %), contactor (burnt contacts, 24 V at the coil), voltage under load (not below the plate minimum, usually 197 V for 208/230 V units), thermostat (batteries, location), drain float (pan full).
7. Oversized system or a very cool day: it hits setpoint in 5–7 minutes and shuts off on the thermostat — that's NOT a failure. The difference: it shuts down on the thermostat's command, everything stops normally, no safety tripped.
8. After the fix: run a full 30+ minute cycle under load, log SC/SH/pressures/amps in the report (Top-off → report) and warn the customer if there's a leak.