Thermostat Calling for Cool but Not Cooling
When a heat pump thermostat calls for cooling but nothing happens — the outdoor unit never starts — the 24VAC control signal is almost certainly making it to the air handler, but something is preventing it from starting the outdoor unit compressor. The diagnostic sequence moves from the thermostat's Y1 output, through the air handler terminal board, out to the outdoor unit contactor coil, then to the compressor capacitor and finally the low pressure safety switch. This guide covers the complete Y1 circuit trace for Carrier, Trane, Lennox, Goodman, Rheem, American Standard, and Amana heat pump systems, and addresses thermostat-side issues for Nest, Ecobee, Honeywell T6/T10, and Emerson Sensi thermostats.
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Common Symptoms
- Thermostat set to COOL and fan is blowing but no cooling occurs
- Indoor air handler fan runs but outdoor unit is completely silent
- House temperature continues to rise despite thermostat calling for cool
- Thermostat display shows it is in cooling mode but compressor is off
- Circuit breaker for outdoor unit appears to be on but unit won't start
Most Likely Causes
- 1
Failed Outdoor Unit Contactor — Coil Open or Contacts Burned
The contactor is a high-current relay in the outdoor unit that receives 24VAC from the Y1 circuit and closes its contacts to supply 240VAC to the compressor and outdoor fan motor. If the contactor coil is open (failed coil), 24V applied to the coil produces no magnetic pull — the contacts never close and the compressor never starts. If the contacts are burned or pitted, they may close but present high resistance, causing voltage drop that prevents the compressor from starting under load.
- 2
Failed Run Capacitor — Compressor Hums but Won't Start
The dual run capacitor provides starting torque to both the compressor and outdoor fan motor. A failed capacitor causes the compressor to hum and draw locked-rotor amperage for a few seconds before the overload protector trips it off. The compressor may attempt to start, then cut out 3–5 seconds later with no airflow from the outdoor unit. On a hot day, this cycle repeats every few minutes as the overload cools and resets.
- 3
Low Pressure Lockout — Refrigerant Issue or Temperature Condition
A low-pressure switch in the refrigerant circuit prevents compressor operation if suction pressure drops below a setpoint (typically 25–40 PSIG on R-410A systems). Low refrigerant charge is the most common cause. Extremely cold outdoor temperatures (below 40°F in cooling mode) can also cause low suction pressure. Some low pressure lockouts require manual reset — others auto-reset after pressure normalizes.
- 4
Outdoor Unit Disconnect Switch Open or Fuse Blown
There is a disconnect box mounted near the outdoor unit — it may contain fuse holders or a simple knife-switch disconnect. A blown fuse or open disconnect removes 240VAC from the contactor contacts, so even if the contactor pulls in on 24V, no line power reaches the compressor. The contactor coil still operates normally, making this look like a contactor or compressor failure.
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Quick DIY Checks
The outdoor unit contactor and capacitor carry 240VAC at high current. Always turn off the outdoor unit breaker at the main panel AND open the outdoor unit disconnect before working inside the outdoor unit cabinet. Verify power is off with a non-contact voltage tester before touching any components.
Run capacitors store a lethal charge even after power is disconnected. Always discharge the capacitor through a 20,000-ohm resistor (do not short it directly) before handling. Never touch capacitor terminals with bare hands.
- 1Verify the outdoor unit disconnect is closed and fuses are intact. Go to the disconnect box mounted on the wall near the outdoor unit. Open the cover — if it's a pull-out style disconnect block, remove it and inspect both fuses with a multimeter set to continuity mode. A blown fuse shows no continuity. Replace fuses with the same amperage rating (typically 30A or 40A for residential heat pumps) and re-insert. If it's a breaker-style disconnect, reset it. Also check the outdoor unit's 240V breaker at the main electrical panel.
- 2Measure 24VAC at the Y1 terminal on the air handler terminal board during a cooling call. Set the thermostat to COOL at 5°F below current room temperature. Access the air handler control compartment. Set a multimeter to AC voltage and measure between Y1 and C (common) terminals on the terminal board. You should read ~24VAC. If you read 0V at Y1 on the air handler board even though the thermostat is calling for cool, the Y1 wire has a break or the thermostat is not outputting on Y1 — verify at the thermostat's Y1 terminal directly. On Nest, Ecobee, Honeywell T6/T10, and Sensi: the Y1 terminal is where the compressor call wire connects.
- 3Trace Y1 voltage to the outdoor unit contactor coil. If Y1 reads 24V at the air handler board, that same signal travels via the Y1 wire to the contactor coil in the outdoor unit. At the outdoor unit control board or contactor terminals, measure 24VAC across the two small coil terminals on the contactor. Should read ~24V during a cooling call. If 24V is present at the coil and the contactor is not pulling in, the coil has failed — replace the contactor.
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Try Pro — $7.99/mo- 4Test contactor pull-in. With 24V applied to the coil, a functioning contactor makes a distinct 'click' and the main contacts close. Measure voltage across the contactor's main contacts (the large terminals) when the coil is energized: a pulled-in contactor should read less than 2VAC across its closed contacts (a small drop due to contact resistance). If you measure 240VAC across the contacts with the coil energized, the contacts are not closing — replace the contactor. Normal closed-contact voltage drop should be under 2V.
- 5Test the run capacitor. With power off, discharge the capacitor by placing a 20,000-ohm resistor across each set of terminals for 5 seconds. Use a digital multimeter with a capacitance measurement mode (μF setting). Measure across the HERM terminals (compressor) and across the FAN terminals. Compare to the rating printed on the capacitor side label (typically 45+5 μF or 35+5 μF). Capacitance within 6% of rated is acceptable. Capacitance more than 10% below rating or a reading of 0 means the capacitor has failed — replace it. On Carrier and Trane units: the dual capacitor is mounted in a plastic bracket inside the outdoor unit panel.
- 6Check for low pressure lockout. Some heat pump outdoor units have a red LED or fault code indicator on the control board. A low pressure lockout typically flashes a specific code (e.g., 3 flashes on Goodman boards, 'LP' on Rheem boards). To attempt a manual reset: turn the thermostat to OFF, flip the outdoor unit breaker off for 30 seconds, then restore power. Wait 5 minutes for the 5-minute anti-short-cycle time delay, then call for cool again. If the compressor starts but trips on low pressure within minutes, low refrigerant charge is indicated — requires an HVAC technician with gauges.
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Repair vs Replace
Contactors and capacitors are the two most common heat pump outdoor unit failures and are among the least expensive repairs — often under $50 in parts for DIY replacement. Low refrigerant requires a technician but a recharge is typically $150–$300. None of these repairs justify replacement unless the compressor itself has failed (seized or shorted windings).
Est. Repair Cost
$15–$50 for capacitor, $25–$80 for contactor
Est. Replacement Cost
$4,000–$8,000 for a new heat pump system
Recommended Tools & Parts
- Buy on Amazon →
Dual Run Capacitor (45+5 MFD, 440V)
Replacement dual run capacitor for heat pump compressor and fan motor. Match MFD ratings and voltage to your unit's data plate. 440V capacitors are universal for both 208V and 240V systems.
$15–$40
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Heat Pump Contactor (24V Coil, 40A, 2-Pole)
Replacement contactor for heat pump outdoor unit. 24VAC coil, 40A 2-pole contacts for compressor and fan power. Verify amperage rating matches your existing contactor data.
$20–$60
- Buy on Amazon →
Non-Contact Voltage Tester
Verifies 240V power is off at the outdoor unit before internal work. Detects voltage without direct contact on live wires.
$15–$30
- Buy on Amazon →
Digital Multimeter with Capacitance Mode
Required for capacitor testing (μF measurement), contactor coil resistance, and Y1 control voltage tracing. Any multimeter with capacitance mode works.
$25–$50
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Read guide →Save $150+ on a single service call
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Frequently Asked Questions
- How do I test if the contactor is the problem on my heat pump?
- Test the contactor in two ways: (1) Coil test — with power on and thermostat calling for cool, measure 24VAC across the two small coil terminals on the contactor. If you read 24V but the contacts are not pulling in (no click and compressor stays off), the coil is open — replace the contactor. (2) Contact test — with 24V applied and contactor 'pulled in' (energized), measure voltage across the main contact set (large terminals). A properly closed contactor reads less than 2VAC across its contacts. If you read the full 240V supply voltage across closed contacts, the contacts are not making proper electrical connection — replace the contactor.
- Why does my heat pump compressor hum for a few seconds then shut off?
- A compressor that hums briefly then shuts off is almost always a failed run capacitor. The capacitor provides the starting torque the compressor motor needs to overcome static friction and begin rotating. Without that boost, the motor draws locked-rotor amperage, heats up rapidly, and the internal thermal overload protector trips the motor off. After a few minutes of cooling, the overload resets and the cycle repeats. Replace the dual run capacitor — it's the most common heat pump outdoor unit repair.
- What is a low pressure lockout and how do I reset it on my heat pump?
- A low pressure lockout occurs when the refrigerant suction pressure drops below the low pressure switch setpoint (typically 25–40 PSIG on R-410A systems). The low pressure switch opens the Y1 circuit at the outdoor unit, preventing the compressor from running on low refrigerant. To attempt a reset: turn the thermostat off, flip the outdoor unit breaker off for 30–60 seconds (resets the time-delay relay and some pressure switch lockouts), restore power, and wait 5 minutes before calling for cool. If the compressor starts but trips again within a few minutes, low refrigerant charge is confirmed — contact an HVAC technician for refrigerant diagnosis and recharge.
- Should I replace the capacitor myself or call a technician?
- Capacitor replacement is a common DIY repair for homeowners comfortable with basic electrical safety procedures. The key steps are: (1) Turn off the outdoor unit breaker and open the disconnect. (2) Discharge the capacitor through a 20,000-ohm resistor before touching. (3) Photograph all wire connections before removing. (4) Match the replacement capacitor MFD ratings and voltage exactly to the failed unit's label. If you're uncomfortable working around 240V components even with power off, this is a reasonable task for an HVAC technician — typically billed at $150–$250 including the capacitor.