Thermostat Calls Cool Outdoor Unit Does Not Start Decision Tree

Why this matters

This is the most common no-cool call and the one most likely to end with "the float switch was open" if you walk it without a plan. Y signal not reaching the contactor coil is a 5-minute job if you know the four places it dies. Done wrong, you've sold a thermostat, then a control board, then a wire chase, and the float switch under the air handler was full of slime the whole time.

Symptom presentation

Indoor unit responds to thermostat (fan starts on G, heat works). Outdoor unit silent on cool call. Sometimes the customer reports "it was working yesterday." Common on rainy weeks (condensate-related), after a storm (transformer or low-voltage damage), or shortly after a service visit (miswiring or shut-off switch left open).

Quick checks (under 2 min)

Step 1: Pull the thermostat off the base. Touch R and Y together for 30 seconds. If the outdoor unit fires, the thermostat is the problem. If not, the failure is between thermostat and contactor.

Step 2: Pull the access panel on the air handler. Find the float switch (usually a small black device in the condensate line). Pull the float linkage by hand or jump it across with a clip lead. Many calls end here.

Step 3: At the outdoor unit, find the disconnect. Look for the service switch INSIDE the disconnect (some have one, some have a pull-out). On the wall: confirm not switched off. Whole-house surge protectors on the disconnect have been known to trip during storms.

Step 4: Read 24 VAC at the contactor coil. Present = contactor problem. Absent = signal-path problem upstream.

Isolation tree

Branch 1: R-Y jumper at the thermostat sub-base does not start the outdoor unit. Signal is not reaching the contactor. Walk it back from the contactor:

• 24 VAC across the contactor coil with thermostat calling: replace contactor.
• No 24 VAC across the contactor coil: continue upstream.
• 24 VAC on the Y wire at the outdoor unit terminal block but not at the contactor: broken wire or loose terminal inside the cabinet. Visual + tug test.
• No 24 VAC on Y at the outdoor terminal block: failure is in the wire run or somewhere indoors.

Branch 2: Float switch / safety switch is open. The float switch is wired in series with the Y signal. Open float = no compressor. Three causes:

• Clogged condensate line. Vacuum the line at the trap or outdoor termination. Clear the trap. Flush with water; verify drainage to outdoor termination.
• Bad float switch. Wet rags get jammed in the float chamber; switches stick. Test continuity with float manually raised and lowered. Replace if it sticks.
• Slope problem causing water to back into the float chamber. Re-slope the condensate line at 1/8 in. per ft minimum per OEM and IMC requirements.

Branch 3: Low-voltage transformer dead.

• 24 VAC on the secondary of the indoor transformer: not the problem.
• 0 VAC on the secondary, 120 or 240 VAC on the primary: replace transformer. Look for the cause of failure before installing the new one - a shorted thermostat wire, a pinched thermostat cable in a recently insulated attic, or a failed control board can take out the new transformer in seconds.
• 0 VAC on the secondary, 0 VAC on the primary: control board, breaker, or fuse problem upstream.

Branch 4: Low-voltage fuse on the indoor control board is blown. Many modern furnaces and air handlers carry a 3 amp glass fuse on the low-voltage side of the board. If it's blown, the system has dropped 24 VAC entirely. Replace the fuse, then find the short:

• Pull all thermostat wires from the board, install the fuse, energize, and add wires back one at a time until the fuse blows.
• Common short locations: outdoor unit terminal block (water intrusion), attic thermostat cable (rodent chew or staples through the cable), a thermostat wire pinched behind a recently installed appliance.
• Do not replace the fuse a second time without finding the short. Repeated fuse replacement burns out the transformer.

Branch 5: Thermostat dead or miswired.

• Smart thermostat without a C wire on a system that previously had a power-stealing thermostat - the new thermostat may not be energizing R correctly. Install a C wire or a C-wire adapter (Venstar, FastStat).
• Programmable thermostat in cool mode but setpoint above current temperature.
• Battery-dead on a battery-powered model with no R input.
• Failed thermostat - swap with a known-good unit and retest.

Branch 6: Outdoor unit control board (modern systems) not energizing the contactor. On communicating systems (Carrier Infinity, Lennox iComfort, Trane ComfortLink) the indoor and outdoor boards talk over a 4-wire bus. A miscommunication or fault code on the outdoor board may suppress the contactor call even with Y energized. Pull the board fault history, address the specific fault per OEM service manual, retest.

Branch 7: Anti-short-cycle delay timer engaged. If the system tripped a safety recently or the breaker was just cycled, the outdoor board may hold off for 3 to 5 minutes before allowing the contactor to close. Wait, retest. Do not jumper the delay.

Confirming the diagnosis

After repair, run a complete cooling cycle from a satisfied state:

• Thermostat calls cool, contactor pulls in within expected delay.
• 24 VAC measured across contactor coil during call.
• Compressor and condenser fan start.
• Float switch tested: lift the float manually; verify outdoor unit shuts down.
• Reset to normal, confirm next call also works.

Repair / remediation

• Float switch: clear condensate line first (vacuum + flush), replace switch only if it fails the continuity test after the line is clean.
• Condensate line re-slope: 1/8 in. per ft minimum; primary line piped to approved termination per IMC 307.
• Transformer replacement: match VA rating (residential typically 40 VA), match primary voltage, mount per OEM.
• Contactor: match coil voltage, pole count, FLA rating.
• Wire repair: any damaged thermostat cable section gets replaced end-to-end, not spliced. Splices in attics and crawl spaces fail later.

References

• International Mechanical Code (IMC) Section 307 - Condensate Disposal
• NEC Article 725 - Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits
• ASHRAE Handbook - HVAC Systems and Equipment
• OEM thermostat wiring guides (Honeywell, Ecobee, Nest, Carrier, Lennox, Trane)