Why Indoor Fan Runs But Outdoor Condenser Does Not

Common Symptoms

• Indoor blower runs normally — no airflow restriction
• Warm air from supply vents despite thermostat set to cool
• Outdoor unit completely silent — no compressor, no fan
• Thermostat displays cooling call (snowflake or COOL indicator)
• No tripped breaker at main panel
• AC runs for a few minutes then outdoor unit shuts off (short cycling)
• Blower runs but no cooling — system just circulates warm air

Most Likely Causes

1. 1

   Float Switch Open — Condensate Pan Full (Very Common in Summer)

   The float switch wires in series with the Y (cooling) wire in the 24V control circuit. When the condensate pan fills, the float opens the Y circuit and shuts off the outdoor unit — but the G (fan) circuit remains active, so the blower continues running. This is the most common cause of this exact split symptom during humid weather. Check the drain pan for standing water first.

2. 2

   No 24V at Contactor Coil — Y Circuit Open

   Set your multimeter to 24V AC. Test between the two small low-voltage terminals on the contactor coil in the outdoor unit. If there's no voltage with the thermostat calling for cool, the Y signal never reached the contactor. Work backward: test at the air handler Y terminal, then at the disconnect, tracing where the 24V disappears. An open float switch, broken wire, or bad thermostat Y connection will show as zero volts at the coil.

3. 3

   Failed Contactor

   If 24V is present at the contactor coil but the outdoor unit still doesn't run, the contactor itself has failed — either the coil is open-circuit (won't pull in) or the contacts have failed open (won't pass line voltage to the compressor). With the thermostat calling for cool and 24V confirmed at the coil, use a multimeter to test line voltage at the contactor load terminals. No voltage = contacts not closing, replace the contactor.

4. 4

   Tripped High-Pressure or Low-Pressure Lockout

   Modern outdoor units have high-pressure and low-pressure switches that lock out the compressor on fault conditions (dirty condenser coil, refrigerant leak, overcharge). These lockouts open the Y circuit at the outdoor unit, so the contactor never pulls in. Look for a lockout indicator on the outdoor control board, or check for a manual reset pressure switch near the refrigerant lines.

5. 5

   Outdoor Disconnect Off or Fuses Blown

   The outdoor disconnect box (mounted on the wall near the condenser) contains cartridge fuses on most systems. A blown fuse kills power to the contactor and compressor but doesn't affect the 24V signal — the blower continues running. Always check the outdoor disconnect fuses first: pull the disconnect block and test each fuse with a multimeter (should read 0 ohms / continuity). Replace with correct amperage fuses if blown.

Quick DIY Checks

1. 1Check the condensate drain pan under the air handler — if water is visible, the float switch has tripped the Y circuit. Clear the drain (see Clogged AC Drain Line article) and the outdoor unit should restart on the next cooling call. This resolves 40%+ of this symptom during peak summer months.
2. 2Pull the outdoor disconnect block (disconnect the power first using the breaker or the pull-out block). Test the cartridge fuses with a multimeter set to resistance. Good fuse: 0 ohms (continuity). Blown fuse: OL. Replace blown fuses with the same amperage rating. If fuses blow again immediately after replacement, the compressor has a fault — do not continue replacing fuses.
3. 3With the system calling for cool, set your multimeter to 24V AC. Test at the contactor coil terminals in the outdoor unit — the two small wires (low-voltage side). Normal: 24V present with a cooling call. Zero volts: the Y signal is not reaching the contactor. If 24V is present, the contactor coil or contacts have failed.

1. 4If 24V is at the coil: listen for a click from the contactor when the system calls for cool. A click means the coil is pulling in. Then test line voltage at the load terminals (large terminals, top and bottom of contactor). If 240V passes through, the outdoor unit has a separate fault (compressor lockout, capacitor failure). If no 240V passes through after the click, the contacts are not closing — replace the contactor.
2. 5If no 24V at the contactor coil: trace backward. Test 24V at the air handler Y terminal (the Y terminal on the control board or terminal strip). If 24V is present there but not at the condenser, check the thermostat wire running between the two units — look for a cut, pinched, or disconnected Y wire. A float switch wired in series with Y will also show zero volts at the contactor when tripped.

Repair vs Replace

Recommended Tools & Parts

• Single-Pole Contactor 24V Coil (30A or 40A, match nameplate)

  Direct replacement contactor for most residential condensers. 24V AC coil, 30A or 40A contact rating at 240V. Match the pole count and amperage to the outdoor unit nameplate.

  $15–$35

  Buy on Amazon →

• Outdoor Disconnect Cartridge Fuses (match amperage — 30A or 60A)

  Time-delay cartridge fuses for residential HVAC outdoor disconnects. Match the amperage on the existing fuse exactly. Time-delay rated for motor loads.

  $5–$15

  Buy on Amazon →

• Mini Float Switch (24V, for condensate drain pan)

  Safety float switch that interrupts the Y circuit when the condensate pan fills. Install in the primary or secondary drain pan to prevent water overflow.

  $10–$20

  Buy on Amazon →

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

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