Thermostat Miswires and Control-Circuit Faults (R, C, Y, G, W, O/B Gotchas)

What it is

Most no-heat and no-cool calls that start right after a thermostat swap aren't equipment failures — they're wiring mistakes: a wire on the wrong terminal, a missed jumper, power "stolen" through a load, or a shorted low-voltage circuit. This article is the field reference for those control-circuit faults — what the symptom looks like, what's wrong, and how to fix it. It assumes you know what each terminal does (R is power, C is the return, Y is cooling, G is fan, W is heat, O/B is the heat-pump reversing valve) and focuses on what goes wrong.

How it works

The 24V control circuit is a simple loop: the transformer puts out 24V, the thermostat switches that 24V out to whatever load it wants on (contactor coil, gas valve, fan relay, reversing valve), and the circuit returns through the common side. Every miswire is a violation of that loop — shorting hot to common, leaving the common disconnected, landing the reversing-valve wire wrong, missing a jumper, or crossing a load wire to the wrong terminal. Diagnosing is just figuring out which part of the loop is broken, and the symptom usually tells you which terminal to look at.

In the field

The universal first move: measure R to C at the thermostat. That's your control power.

• ~24–28V AC R-to-C → you have control power; the fault is downstream (a specific load circuit or a wrong landing).
• ~0V R-to-C → no control power getting to the stat: blown low-voltage fuse, dead transformer, tripped furnace door switch, or a broken/shorted wire. A blown fuse points hard at a short in the thermostat wiring.

From there, chase the specific circuit by the symptom (below). Always verify wire function at both ends — the colors lie; only the landing is truth. A "red" wire on G is a fan wire.

Common faults & what they mean

Blowing the low-voltage fuse (or board fuse) the moment you power up

• Cause: R shorted to C somewhere — most often a chafed thermostat cable stapled through at the cabinet, a pinched wire behind the stat, or a wire bridging two terminals.
• Fix: find the short, don't upsize the fuse. The fuse protects the transformer and wiring. Disconnect the thermostat wires at the board and ohm/inspect the cable; if the fuse holds with the stat disconnected, the short is at the stat or in the cable.

Smart/Wi-Fi thermostat keeps rebooting, won't hold settings, or drops Wi-Fi

• Cause: missing or bad C wire. The stat is "power stealing" through a load to charge itself and browning out under load. Classic on a stat that worked as a dumb stat but resets as a smart one.
• Fix: land a true common — use a spare conductor on C at both the stat and the equipment, add an add-a-wire/C-wire adapter, or run new cable. Don't rely on power-stealing for a modern stat.

Heat pump heats when you call cool / cools when you call heat

• Cause: O/B mismatch — the reversing-valve wire is on the wrong terminal, or the stat is configured for the wrong one (O energizes the valve in cooling on most brands; B energizes it in heating on others). The system uses one or the other.
• Fix: confirm which the equipment uses (check the unit/board), land the reversing-valve wire on the matching terminal, and set the stat's O/B configuration to match. Don't land both O and B.

Fan runs continuously and won't shut off

• Cause: G energized when it shouldn't be — fan switch left in ON, G jumped/shorted to R, or a stuck fan relay.
• Fix: set fan to AUTO; if it still runs, check for G shorted to R at the stat/board and check the fan relay.

No cooling, but the indoor fan runs fine

• Cause: the Y circuit is open or unpowered — Y not landed, Y wire broken, the contactor coil open, or a safety in the Y path (high-pressure or condensate float switch) tripped and interrupting Y.
• Fix: confirm R-to-Y calls 24V on a cool demand, then follow Y outdoors. A tripped float or high-pressure switch in series with Y kills cooling and looks exactly like a thermostat problem.

No heat, fan and cooling work

• Cause: the W circuit is open — W not landed or broken, or the heat source (gas valve / sequencer / aux relay) isn't getting the call. On a heat pump, aux/W2 or emergency heat may be miswired.
• Fix: confirm R-to-W energizes on a heat call, then follow W into the heat source.

Only heat works, or only cool works — never both

• Cause: the two-transformer trap. On a single-transformer system the stat needs the Rh/Rc jumper so one R feeds both sides. Without it, only the side power landed on works. On a genuine two-transformer system, the jumper must be removed or you tie two transformers together and smoke one.
• Fix: one transformer → install the Rh/Rc jumper (or use the single R terminal). Two transformers → remove the jumper and land Rh and Rc on their own power.

New stat installed, nothing works (no heat, no cool, no fan), R-to-C reads 0

• Cause: lost the common path, the blower door switch is open, the low-voltage fuse blew during the swap (a momentary R-to-C touch), or R isn't landed.
• Fix: reseat the blower door, check the fuse, and verify R and C are landed at both ends.

Tech tips & gotchas

• Measure R-to-C first, every time. It splits the problem in half instantly: power present (downstream fault) vs no power (transformer/fuse/door/short). Don't pull wires before you know which half you're in. And a blown fuse means a short — find it, don't just replace it, or you cook the transformer next.
• A tripped safety in the Y path mimics a thermostat problem. A condensate float switch or high-pressure switch in series with Y kills cooling with the stat working perfectly. Check the safeties before chasing the stat.
• Suspect config before you condemn a part. Heat pump running backwards after a swap = O/B config, not a bad reversing valve. Wi-Fi stat resetting = missing C, not a bad stat. Only-heat-or-only-cool = the Rh/Rc jumper.
• Function over color, both ends, and photo the old wiring first. Whoever ran the cable used whatever conductor was handy — the only truth is what's landed where. A five-second photo saves a callback when the new stat's terminals don't map one-to-one.

Safety / code notes

• The 24V control side is Class 2 low voltage — it won't shock you, but the transformer's line side is full 120/240V; kill power at the disconnect before working in the cabinet.
• A shorted low-voltage circuit will repeatedly blow the board/transformer fuse — find and fix the short; the fuse protects the transformer and wiring, so don't upsize it. Class 2 control wiring still has to be installed and protected per the applicable electrical-code provisions and routed where it can't chafe against sheet metal or moving parts.
• After any control rework, verify correct operation in every mode (heat, cool, fan, and — on a heat pump — emergency/aux) before you leave. A reversed O/B or a miswired aux can run the equipment wrong without obviously failing.