A/C Diagnostics
The Gauge Playbook
Every low-side/high-side combination, what actually causes it, and what to check next. Readings shown assume ~85°F ambient, ~1500 rpm, A/C max, doors open — always judge pressures against ambient temperature, not a fixed number. Charge weights for your exact vehicle are on its Fluids tab.
✅ Normal system
Low ~25–45 psi · High ~180–260 psi (at ~85°F ambient)
Likely causes (in order):
- System healthy
Vent temps 38–48°F. High side tracks ambient: rough rule, high-side ≈ 2.2–2.5× ambient °F. Hotter day = higher numbers — judge against ambient, not a fixed chart.
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🩸 Both sides LOW
Low 10–25 psi · High 100–160 psi, weak cooling
Likely causes (in order):
- Undercharge (leak — the #1 cause)
- Severely restricted airflow through evap (rare)
Find the leak before adding refrigerant: UV dye or electronic sniffer at compressor shaft seal, condenser (rock strikes), rusty spring-lock/line fittings, evap drain (oily residue = evap leak). Charging a leaker just rents cold air.
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🔥 Both sides HIGH
Low 50–80 psi · High 300–400 psi, poor cooling, may cut out
Likely causes (in order):
- Overcharge (most common after a DIY top-off)
- Condenser airflow: dead fan, blocked/bug-packed condenser
- Air/moisture in system (never pulled vacuum)
Fans running full speed with A/C on? Condenser packed with debris? If airflow is good, recover, pull deep vacuum 30+ min, recharge to the EXACT label weight — this is why charge specs are on every vehicle page here.
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🫀 LOW high · HIGH low (converging)
Low 60–80 psi · High 120–170 psi — the two needles “meet in the middle”
Likely causes (in order):
- Weak/failing compressor (not building differential)
- Variable-displacement compressor stuck at minimum stroke
- Blown internal compressor seals
Compressor engaged and turning (not just clutch spinning free)? On variable-displacement systems check the control valve/commanded stroke with a scan tool before condemning the compressor.
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🧊 LOW into vacuum · HIGH low-normal
Low 0 to vacuum · High 100–180 psi, little/no cooling
Likely causes (in order):
- Restriction: clogged orifice tube or expansion valve stuck closed
- Plugged receiver-drier/accumulator
- Line kinked or iced internally (moisture freezing at the metering device)
Feel the lines: a restriction makes a sharp temperature break RIGHT AT the blockage (frost/sweat line). Desiccant debris in the orifice screen = a failing drier or compressor shedding material — look further upstream.
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💧 LOW high · HIGH normal
Low 50–65 psi · High normal-ish, cooling mediocre
Likely causes (in order):
- Expansion valve stuck OPEN (flooding the evap)
- Orifice system: mis-sized/missing orifice tube after a repair
Evap flooding = suction line sweating/frosting back to the compressor. On expansion-valve systems, check bulb contact/insulation on the suction line before replacing the valve.
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🔁 Rapid clutch cycling
Low swings fast (e.g. 22→45 psi) as clutch kicks on/off every few seconds
Likely causes (in order):
- Low charge (pressure switch protecting the compressor) — most common
- Faulty cycling/pressure switch
Short-cycling with mediocre cold = classic slightly-low charge. Verify with gauges against the label weight rather than “adding a can until cold.”
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⛔ Compressor won’t engage (both sides equal, static)
Both gauges equal at static pressure (system equalized, ~ambient-dependent)
Likely causes (in order):
- Low-pressure lockout (charge too low to close the switch)
- Clutch relay/fuse, clutch coil, excessive clutch air gap
- ECM disabling A/C (engine temp, WOT input, control head request missing)
Static pressure ≈ saturation pressure for the ambient (e.g. ~75–90 psi at 80–90°F with R-134a) means there IS charge — chase the electrical side: command the clutch with a scan tool, check power at the coil connector.
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🌡️ HIGH climbs at idle, drops when driving
High side 300+ psi at idle, normalizes with vehicle speed
Likely causes (in order):
- Cooling/condenser fan not running (or wrong speed)
- Condenser fins packed with debris
The freeway is doing the fan’s job. Watch the fan the moment high-side passes ~250 psi at idle — no fan = your answer.
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🧊 Cools great… then airflow dies off
Starts normal; low side drifts low; vents strong→weak while temp stays cold
Likely causes (in order):
- Evaporator freezing over: cycling switch/evap temp sensor failed
- Expansion valve overfeeding + control fault combo
- Blend door/recirc masking as freeze-up
Shut A/C off, fan on high — airflow returns in 10–20 min as the ice melts = confirmed freeze-up. Then chase the control that should have cycled the compressor.
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The two rules that prevent 80% of A/C comebacks: (1) Charge by weight to the label spec — never “to the gauges,” never “by cans” (your vehicle's exact ounces are on its Fluids tab here). (2) Orifice-tube systems and expansion-valve systems read differently on gauges — know which one you're holding before you condemn parts. Per-vehicle system types are rolling out on every A/C spec page.
Chased everything and it still isn't right? That's what the Diag Desk is for — a vetted master tech looks at YOUR readings on YOUR vehicle. $25, free if unanswered in 24h.