Goodman AC Not Cooling: Step-by-Step Diagnosis

Common Symptoms

• AC runs but house stays at 80°F or above despite hours of runtime
• Condenser fan spinning but compressor humming without starting
• Outdoor unit completely silent when thermostat calls for cooling
• Ice on the suction line or on the indoor evaporator coil
• Intermittent cooling — works for a while then stops, restarts after a rest
• Reduced airflow from vents despite blower running
• High electric bill with poor cooling performance

Most Likely Causes

1. 1

   Failed Dual-Run Capacitor — Very Common on Goodman (First Check)

   Capacitor failure is the most common repair call on Goodman outdoor units — field experience shows Goodman's OEM capacitors fail earlier than premium brands in hot climates. The dual-run capacitor serves both the compressor (HERM terminal) and the condenser fan motor (FAN terminal). When the HERM section weakens, the compressor draws excessive startup current, trips its thermal overload, and shuts off — giving you the 'cools for 30 minutes then stops' pattern. A weak FAN section shows as a slow-spinning or non-spinning condenser fan. On GSX16 and GSX18 units, the capacitor is mounted in the electrical compartment on the side of the outdoor unit. Test in capacitance mode — replace if more than 6% below the rated MFD. The MFD and voltage rating are on the capacitor label and unit data plate.

2. 2

   Contactor Pitting — Common on Older Goodman Units

   Goodman contactors are a known weak point on units over 8–10 years old. The contact faces accumulate arc erosion faster than premium brands, developing pitting that causes high contact resistance. Pitted contacts may still conduct but create a voltage drop that starves the compressor and causes it to overheat. A fully open contactor coil means no contact closure at all: the thermostat sends a 24VAC signal, but neither fan nor compressor starts. Test by measuring 24VAC at the contactor coil terminals during a cooling call — voltage present but contactor not pulling in means coil failure. On GSX-series units, the 2-pole contactor is located in the electrical compartment with the capacitor.

3. 3

   Refrigerant Leak at Schrader Valve Cores

   Goodman units have a specific pattern of refrigerant loss at the Schrader valve cores on the high-side and low-side service ports. The Schrader core is a removable valve insert (similar to a tire valve stem) that can develop a small leak — a slow weep of refrigerant over months or years. Signs: gradually worsening cooling performance over a season or two, slight oil staining around the service port caps, or ice forming on the suction line. A licensed technician can test the Schrader cores with leak detection soap or an electronic leak detector, replace the cores ($2–$5 per core), and recharge the system. The fix is inexpensive if found before the charge has leaked down entirely. Refrigerant work requires EPA 608 certification.

4. 4

   Coil Icing — Dirty Filter or Low Refrigerant

   Ice on the suction line at the outdoor unit or on the indoor evaporator coil means the evaporator is running below freezing. Two causes: (1) dirty air filter blocking airflow — the coil can't absorb enough heat from the air and drops below freezing, or (2) low refrigerant charge — the reduced pressure in the evaporator circuit drops below the freezing point of the refrigerant. Diagnose by replacing the filter first. If ice returns within one hour of thawing with a fresh filter, you have a refrigerant issue. Shut the unit off when you see ice — running a system with a frozen coil floods the compressor with liquid refrigerant and causes compressor failure.

5. 5

   Dirty Condenser Coil

   Goodman condenser coils use standard aluminum fin-and-tube construction. Cotton from cottonwood trees, grass clippings blown by lawn mowers, and pet hair pack into the fin array and insulate the coil from air. A fouled coil raises condensing pressure and reduces cooling capacity — on very dirty units, the compressor may trip its thermal overload from overheating. Cleaning the coil with a garden hose (inside-out direction, gentle fan spray) is a free fix that takes 15 minutes and can restore full cooling capacity.

Quick DIY Checks

1. 1Step 1 — Filter and thermostat check: confirm thermostat is in COOL mode, fan on AUTO, set point at least 5°F below room temperature. Check the air filter — pull it from the return air slot on the air handler or from the wall/ceiling return grille. If it's gray or matted, replace it before anything else. A clogged filter is the cheapest and most common cause of poor cooling. After replacing the filter, go outside and confirm the outdoor unit is attempting to run.
2. 2Step 2 — Outdoor unit observation: listen and look at the condenser. Fan spinning on top? Compressor humming? (a) Fan and compressor both running — unit operating, check refrigerant and coil cleanliness; (b) Fan running, compressor humming 3–5 seconds then stopping — HERM capacitor section failed; (c) Fan slow or not spinning, compressor running normally — FAN capacitor section failed; (d) Neither running, no response to thermostat — check contactor and 24V signal.
3. 3Step 3 — Capacitor test (MFD reading): shut off the outdoor disconnect box. CRITICAL: wait 5 full minutes before touching any internal components — the capacitor holds 370–440VAC even after power is cut. Remove the electrical access panel (4–6 screws on the side of the cabinet). The dual-run capacitor is the cylindrical metal can with HERM, FAN, and C terminals. Set your multimeter to capacitance (µF or MFD) mode. Disconnect one wire at a time and test HERM-to-C, then FAN-to-C. Compare each reading to the label on the capacitor. More than 6% below rated MFD on either section means replace. Goodman units typically use 35+5 or 45+5 MFD at 370V or 440VAC — match your replacement exactly.

1. 4Step 4 — Contactor check (voltage in vs out): with the disconnect off, inspect the contactor contact faces for pitting or carbon buildup. Pitting across more than half the contact face means replace the contactor. With power restored and the thermostat calling for cooling, use a multimeter on 250VAC range to measure across the contactor coil terminals (the two small wires — not the large line-voltage terminals). You should read 24VAC when the thermostat is calling for cooling. If you have 24VAC at the coil but the contactor isn't pulling in, the coil has failed. Also test line-voltage input and output: measure across the LINE-side terminals (large terminals at top) — should read ~240VAC. Then measure across the LOAD-side terminals (large terminals at bottom, going to compressor) — should also read ~240VAC when contactor is closed. Zero volts on the load side with contactor pulled in means severely pitted contacts with no through-continuity.
2. 5Step 5 — Refrigerant pressure signs: with the unit running, feel the large suction line (the insulated copper line running between the outdoor and indoor units) near the outdoor unit. It should feel cold to the touch — significantly colder than ambient air. Also check for ice formation on the suction line or at the outdoor unit's service port area. Look at the service ports for oil staining or residue around the caps — this is a sign the Schrader cores may be leaking. If you find oil around service ports, unscrew the caps and look for a light coating of oil on the Schrader core stems — this confirms a slow leak. Refrigerant leak repair requires an EPA 608 licensed technician.
3. 6Step 6 — Condenser coil cleaning: with the unit off, rinse the condenser coil fins with a garden hose set to a gentle fan spray, directing water from inside the outdoor unit outward through the fins to push debris out. Work around all accessible sides of the cabinet. Never use a pressure washer — it bends the aluminum fins and reduces airflow. After cleaning, restore power and allow the unit to run for 10 minutes before evaluating cooling performance. A clean coil on a 95°F day should keep high-side pressure in the 350–450 PSI range on R-410A systems.

Repair vs Replace

Recommended Tools & Parts

• Goodman AC Dual-Run Capacitor (35+5 or 45+5 MFD 440V)

  Replacement dual-run capacitor for Goodman GSX, DSXC, GSXC, and related outdoor condenser units. Always verify the exact MFD ratings (HERM and FAN sections) and voltage from the label on your existing capacitor before ordering. Common Goodman residential ratings: 35+5 MFD and 45+5 MFD at 370V or 440V. Fixes compressor-won't-start, hard-start, and condenser fan slow or stopped symptoms.

  $15–$45

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• Goodman 2-Pole 40-Amp Contactor

  Replacement 2-pole contactor for Goodman and Amana outdoor condenser units. Match the amperage (typically 30A or 40A) and coil voltage (24VAC) to your existing unit's data plate. Fixes units with neither fan nor compressor starting despite thermostat calling for cooling, and intermittent no-cooling from pitted contacts.

  $18–$40

  Buy on Amazon →

• MERV 11 Pleated Air Filter

  High-efficiency pleated air filter for central AC and furnace systems. Replace every 1–3 months — a clogged filter is the leading cause of evaporator coil icing and reduced cooling capacity on Goodman systems.

  $8–$20 per filter

  Buy on Amazon →

• Supco SPP6 Hard Start Kit

  Capacitor-style hard start kit for struggling compressors. Adds extra starting torque to compressors that are slow to start or that require multiple attempts. Particularly useful for older Goodman compressors with tight tolerances. Easy to install across the HERM and C terminals alongside the existing capacitor.

  $25–$45

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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