Central air conditioning wiring at the thermostat is four terminals: R (24V power), C (common), Y (compressor), and G (fan). No W, because a cooling-only system has no heat call. If your AC stopped working right after you installed a new thermostat, the fix is almost always here: a missing jumper, a loose Y wire, or a safety switch that cut power on purpose.
This page covers central split-system AC wiring start to finish. For the full terminal reference across every system type, see the thermostat wiring guide.
TL;DR
| Wire (typical color) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc) | 24V AC power from the transformer |
| Blue (or black) | C | Common, powers a smart thermostat |
| Yellow | Y | Cooling call, closes R to Y and energizes the compressor contactor |
| Green | G | Fan relay, runs the indoor blower |
No W terminal on a cooling-only system. If your thermostat has a W wire too, you have heat as well; see the combined heat + AC section below. This page is about central split systems (indoor air handler or furnace, outdoor condenser), not window units.
What “AC wiring” means on a central system
A central air conditioner is two boxes connected by refrigerant lines and a low-voltage control cable: an indoor unit (furnace or air handler) and an outdoor condenser holding the compressor and a fan. The thermostat never touches the compressor directly. It sends a 24V signal down the Y wire to the indoor control board, which relays that signal to the condenser to energize the compressor contactor.
That’s the whole job of cooling-only wiring: R for power, C for common, Y to start the compressor, G to run the fan.
AC color code
| Wire color (typical) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc / Rh) | 24 V AC power from the transformer. Rc = cooling power, Rh = heating power; one system uses a jumper between them. |
| White | W (W1, W2) | Heat call. W2 is second-stage or auxiliary heat. |
| Yellow | Y (Y1, Y2) | Cooling call — energizes the compressor contactor. On heat pumps, Y runs the compressor for heating too. |
| Green | G | Indoor fan (blower) relay. |
| Blue | C | Common — completes the 24 V circuit and gives smart thermostats constant power. Sometimes black. |
| Orange | O (or O/B) | Heat-pump reversing valve, energized in cooling (most brands). |
| Dark blue | B | Heat-pump reversing valve, energized in heating (Rheem/Ruud). Occasionally used as C — verify. |
| Brown | E | Emergency heat — manual backup that locks out the compressor. |
| Varies | AUX | Auxiliary heat — automatic electric backup on heat pumps. Often shares a wire with W2. |
The table above is the convention, and on a cooling-only system it usually holds: red to R, blue to C, yellow to Y, green to G. But colors are whatever the installer had on the truck that day, and Honeywell’s own support documentation says there’s no enforced standard. Go by the terminal letter printed on the old thermostat’s sub-base, not the color of the insulation, and photograph the wiring before you disconnect anything.
How Y reaches the compressor
This is the part most wiring guides skip, and it’s the key to diagnosing “AC stopped after I installed a new thermostat”:
- You call for cooling. The thermostat closes an internal switch between R and Y.
- 24V flows down the Y wire to the indoor control board.
- The control board relays that signal out to the condenser over the low-voltage cable.
- At the condenser, 24V energizes the compressor contactor coil, an electromagnet that pulls a set of heavy contacts closed.
- The closed contactor connects the compressor and outdoor fan motor to line-voltage power (240V, from the breaker, not the thermostat), and the system starts cooling.
Every step depends on the Y wire staying connected and carrying 24V. A loose screw, a corroded splice, or a jumper you forgot to remove breaks the chain anywhere along it, and the symptom looks the same at the thermostat: it calls for cooling and nothing happens outside.
How to wire a cooling-only thermostat, step by step
Safety first: turn off power at the breaker or the air handler’s service switch, and confirm the thermostat display is dark before pulling it off the wall. The control side is only 24V AC, but shorting two wires can blow the control-board fuse or kill the transformer. Never touch anything at the outdoor condenser with power on; that side carries 240V line voltage.
- Kill the power. Breaker for the air handler/furnace, or the service switch on its cabinet. Confirm the display is dead.
- Photograph the wiring. One wide shot of the whole sub-base, one close enough to read the terminal letters.
- Label each wire by letter. Masking tape flags marked R, C, Y, G, not colors.
- Check for a jumper. A metal strap or short wire bridging two terminals (commonly R to Rc, or Rc to Rh) usually needs to stay behind on the old base if the new thermostat bridges that internally. See the Rc/Rh jumper guide if your system shows both Rc and Rh.
- Disconnect and secure the cable. Unscrew each terminal, then clip or tape the cable bundle to the wall plate so it can’t slide back into the wall cavity.
- Mount the new base and land the wires. Each wire goes to the terminal with the same letter. Seat and tug-test each one. A loose Y wire is an intermittent “calls for cooling, nothing happens” fault that shows up on the hottest day of the year.
- Set the installer menu. System type (cooling only, or heat + cool), fan type, and cycling rate.
- Restore power and test cooling and fan separately. Let the compressor run a full minute before checking that cold air is actually coming out of the registers.
AC stopped working right after a new thermostat: the checklist
This is the single most common support question on cooling-only systems. Work it in order, cheapest and most likely first.
Most common cause: the Rc/Rh jumper is missing on a dual-transformer system.
- Rc/Rh jumper. If your air handler has two separate transformers, one for heating and one for cooling, the old thermostat likely had a jumper bridging Rc to Rh. Some new thermostats bridge this internally; others need the jumper reinstalled. Confirm which situation you have: thermostat Rc/Rh jumper wiring.
- Y not landed, or landed loose. Pull the thermostat and tug-test the Y wire at both the thermostat and the control board.
- Wrong system type selected. If the new thermostat’s installer menu is set for two-stage cooling (Y1/Y2) but your equipment is single-stage, it may never send the call your compressor is wired to receive. Recheck the equipment-type setting.
- Condensate float switch tripped. Air handlers have a safety float switch in the drain pan that cuts power to Y when the pan fills, protecting against an overflow from a clogged drain line. Clear the clog, and the switch usually resets once the pan drains.
- Blown control-board fuse. A short during install can blow the low-voltage fuse on the air handler board. Look for a small blade fuse near the transformer connection; the rating is printed on the board.
If two of these show up at once, re-shoot your wiring against the before photo. The most common install error is two wires swapped at the new base, Y and C landed backward being a frequent one since both are sometimes similar colors.
Window units and portable AC: not this page
A window-mounted or portable air conditioner has its own built-in thermostat and controls. There’s no wall thermostat wiring involved, because there’s no separate condenser and no low-voltage control cable to run. This page covers central split systems only: an indoor furnace or air handler, an outdoor condenser, and a wall thermostat connecting them over 24V control wire. Adding smart control to a window unit is a different product category (a smart plug or window-unit-specific controller), not a wall thermostat wiring job.
Most homes have both: combined heat + AC wiring
Cooling-only R/C/Y/G is common in mild climates, but most homes with central air also have a furnace or heat pump on the same thermostat. Add a W wire and you get the standard five-terminal combo:
- R: 24V power.
- C: common, for constant power.
- W: heat call. R to W fires the furnace.
- Y: cooling call, same as above.
- G: fan.
The Y-to-contactor path works identically whether or not W is present. Full walkthrough of the five-wire combo, including the color table and troubleshooting for both heat and cool: 4-wire thermostat wiring covers the base R/W/Y/G circuit this expands on.
If your outdoor unit also runs in winter, meaning it’s a heat pump rather than a furnace, the wiring uses O/B instead of W and the compressor runs the heating side too. Check that before assuming a straight AC-plus-furnace setup: heat pump thermostat wiring.
Troubleshooting AC wiring problems
| Symptom | Most likely cause | Check |
|---|---|---|
| Thermostat calls for cooling, nothing happens outside | Y wire loose or unlanded, or condensate float switch tripped | Tug-test Y at both ends; check the drain pan and float switch |
| AC ran fine, then died right after a thermostat swap | Missing Rc/Rh jumper, or Y/C swapped at the new base | Rc/Rh jumper check; re-verify wiring against your before photo |
| Compressor hums but doesn’t start, outdoor fan doesn’t spin | Not a wiring issue; likely a failing contactor or capacitor | Beyond the 24V control side; call an HVAC tech |
| Indoor fan runs, but no cold air | Y call isn’t reaching the condenser, or the compressor is locked out | Confirm 24V at the condenser’s low-voltage terminals while calling for cooling |
| Blank display after install | Blown control-board fuse from a short during install | The blade fuse on the air handler board; then measure across R and C (should read ~24-28V AC with the stat off the wall) |
| AC short-cycles or the contactor chatters | Long wire run causing voltage drop, or a failing contactor | Confirm the wire gauge fits the run; see the wire gauge guide; if wiring checks out, the contactor is a pro job |
FAQ
What wires does an AC-only thermostat need?
Four: R (24V power), C (common), Y (compressor/cooling call), and G (fan). There’s no W terminal, because cooling-only systems have no heat call.
Why did my AC stop working right after I installed a new thermostat?
Almost always one of four things: a missing Rc/Rh jumper, a loose Y wire, the wrong system type selected, or a tripped condensate float switch cutting power to Y on purpose. Work the checklist above in order.
How does the thermostat actually turn on my AC compressor?
It closes R to Y. That sends 24V to the indoor control board, which relays it to the condenser and energizes the compressor contactor coil. The contactor then connects the compressor and outdoor fan to their 240V power. The thermostat never switches the compressor’s actual power; it only triggers the contactor.
Can I wire a smart thermostat to my central AC without a C wire?
Often yes, initially. Many smart thermostats power-share through the Y wire when idle, but that can cause the exact symptoms this page covers: relay chatter, phantom starts, or dropouts. See how to install a thermostat without a C wire for permanent fixes.
Is thermostat wiring for a window AC unit the same as this?
No. Window and portable units are self-contained, with their own built-in controls and no wall thermostat.
What if my outdoor unit runs in winter too?
Then it’s a heat pump, not a straight AC-plus-furnace system, and the wiring pattern is different. See heat pump thermostat wiring before wiring it as standard AC.
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
Last reviewed: July 2026. Wiring conventions verified against Honeywell Home and Google Nest support documentation; always confirm against your equipment’s manual.
