Upload a Thermostat Photo for AI Diagnosis

Common Symptoms

• Your HVAC system won't turn on after installing a new thermostat
• The heat pump heats in cooling mode and cools in heating mode (O/B polarity reversed)
• Aux heat never activates in cold weather despite being wired
• Thermostat display is blank or loses power intermittently (missing or incorrect C-wire)
• The system short-cycles — turns on and off every few minutes without reaching set temperature
• Only some functions work: fan runs but heat or cool doesn't, or heat works but cooling doesn't
• You're unsure if your wiring is correct after a thermostat replacement and want verification

Most Likely Causes

1. 1

   Thermostat Terminal Labels Are Inconsistent Across Manufacturers

   The 24VAC thermostat wiring standard uses letters for terminal functions (R for power, C for common, Y for cooling, W for heat, G for fan), but manufacturers implement them inconsistently. Rh and Rc are both power terminals but for different transformers — confusing even for professionals. O energizes a heat pump reversing valve in cooling mode on most brands, but B energizes it in heating mode on Bosch, some Bryant, and some York units. Getting these wrong produces behavior that looks exactly like equipment failure. The AI knows the correct terminal mapping for every major thermostat brand.

2. 2

   C-Wire Requirements Are Invisible Until the Thermostat Loses Power

   Modern smart thermostats (Nest, Ecobee, Honeywell T6/T9, Emerson Sensi) require a continuous 24VAC common (C-wire) to power their Wi-Fi, display, and sensors. Many older systems were installed with only 4-wire thermostats and no C-wire run to the thermostat. A thermostat that works for a few days then goes blank, or that reboots every time the AC starts, almost always has a C-wire problem. The AI Wiring Scan identifies whether a C-wire is present in your photo and flags missing C-wire as an issue if your thermostat model requires it.

3. 3

   Heat Pump O/B Terminal Polarity Is Set-and-Forget — And Often Wrong

   The O/B terminal controls a heat pump reversing valve — the component that switches the refrigerant flow direction between heating and cooling. Most brands (Carrier, Trane, Lennox, Rheem) use O terminal logic: the valve energizes in cooling mode. Bosch and a few others use B logic: the valve energizes in heating mode. Getting this wrong means your heat pump heats when you want cooling and cools when you want heating — a symptom that looks exactly like a failed reversing valve or refrigerant leak. The AI identifies O vs B configuration requirements from your thermostat model and flags incorrect settings.

4. 4

   Rh/Rc Jumper Removal During Replacement Is a Silent Failure Mode

   Standard thermostats ship with a jumper wire between Rh and Rc terminals because most residential systems use a single transformer. When removing an old thermostat, the jumper wire is often discarded with the old thermostat's base plate. If your new thermostat has both Rh and Rc terminals but only one R wire, you must install a short jumper between them — or only one of the two systems (heating or cooling) will get power. The AI identifies missing Rh-Rc jumpers from wiring photos and flags this common installation error.

5. 5

   Multi-Stage and Dual-Fuel Systems Have Additional Terminals That Are Easily Missed

   Two-stage heat pumps have a Y2 terminal for the second compressor stage. Dual-fuel systems (heat pump plus gas furnace) require both the O/B terminal and a W1 or W2 terminal for the gas heat stages. Systems with electric auxiliary strips need an Aux/W2 terminal for cold-weather backup heat and an E terminal for emergency heat mode. Each of these is a separate wire that must be correctly connected. Omitting Y2 means the system never enters second-stage cooling; omitting W2 means backup heat never activates. The AI cross-references your system type description with the wiring photo to flag missing stage connections.

Quick DIY Checks

1. 1Remove the thermostat from its wall plate by pulling it straight off (most snap on). You'll see the wiring terminals on the wall plate with labeled positions. Before doing anything else, take a clear photo of the wall plate wiring — wires connected, labels visible. This photo is what you'll upload to the Wiring Scan at /wiring-scan. Use your phone's flash; the terminal labels are small.
2. 2Also photograph the back of the thermostat itself (the side that connects to the wall plate) so the AI can identify your thermostat model and cross-reference the expected terminal configuration for that specific model. If you're replacing an old thermostat, photograph the old wiring before disconnecting anything — this is your baseline reference.
3. 3Go to /wiring-scan and upload your photos. In the description, note: (1) your thermostat model (found on the front or inside the device), (2) your system type (central AC with gas furnace, heat pump, heat pump with aux heat, dual-fuel, etc.), (3) the specific symptom you're experiencing (won't heat, won't cool, blank display, short-cycles, etc.). The more context you provide, the more accurate the AI diagnosis will be.

1. 4Review the AI findings. For each flagged terminal issue, the results will specify which wire, which terminal, and what the correct configuration should be. For O/B polarity errors, the fix is changing a setting in the thermostat setup menu (not rewiring) on most modern thermostats. For missing C-wire, the AI will recommend options: running a new wire, using a C-wire adapter (like the Ecobee PEK or a Venstar Add-A-Wire), or using a power-stealing option if your thermostat supports it.
2. 524VAC voltage test — after wiring changes, verify the system is getting correct control voltage: set your multimeter to AC voltage (50V range). With the thermostat removed from the wall plate, probe between the R terminal and the C terminal (common) at the wall plate. You should read 24–28VAC. This is the transformer output powering the control circuit. Below 20VAC indicates a transformer problem or excessive load; above 30VAC indicates a potential transformer issue. A reading of 0V on R-to-C with the system powered means the transformer has failed or the C-wire is not connected at the air handler end. Upload a photo of your multimeter and wall plate together to /wiring-scan for an AI interpretation of your voltage reading.
3. 6After making any corrections, reconnect the thermostat and test all modes: set to COOL and verify the outdoor unit runs (you should hear the compressor start within 1–2 minutes). Set to HEAT and verify the furnace or heat pump heats properly. Set the fan to ON and verify the blower runs independently. For heat pumps, set a temperature 5°F below current room temp and verify the system enters cooling mode — if it heats instead, your O/B polarity setting is still incorrect. Use /diagnose to upload a photo of the thermostat display during a call for further AI-assisted verification.

Repair vs Replace

Recommended Tools & Parts

• C-Wire Adapter (Add-A-Wire)

  Converts a 4-wire thermostat installation to 5-wire to add the C (common) wire required by modern smart thermostats. Compatible with most 24VAC systems including single-stage and two-stage AC and heat pumps. Venstar ACC0410 and Footprint Hero are the most reliable options. Installs at the air handler in 15 minutes.

  $20–$35

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• Digital Multimeter

  For verifying 24VAC control voltage at thermostat terminals (R-to-C should read 24–28VAC). Also used to test transformer output, check wire continuity between thermostat and air handler, and verify 24VAC at each terminal. AstroAI AM33D or Fluke 101 are good starting points for HVAC thermostat work.

  $18–$50

  Buy on Amazon →

• Thermostat Wire 18/5 (18 AWG 5-Conductor)

  Standard 18 AWG 5-conductor thermostat wire for running a new C-wire when an add-a-wire adapter isn't practical. Use for new thermostat installations, replacing damaged thermostat cable, or adding a C-wire to systems with only 4-wire cable. Available in 50-foot rolls for most residential applications.

  $15–$30

  Buy on Amazon →

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

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