York TM9V080B12MP11 Error Code 4 Amber Flashes: Y Signal Without G Signal
What Does Code 4 Amber Flashes Mean?
The board sets this code when it sees a "Y" call (for cooling) arrive without the accompanying "G" signal that turns on the indoor blower. In a correctly wired system a cooling call sends both: Y starts the outdoor unit and G runs the furnace's blower to move conditioned air. Seeing Y without G tells the board there is a thermostat wiring gap, so it posts this alert.
Importantly, this is a low-severity notice, not a shutdown. The manual states the furnace will still operate normally in both heating and cooling; the code exists only to flag the wiring problem so it can be corrected. The most common cause is a G wire that is loose, disconnected, or missing at the thermostat terminal or at the furnace control board's low-voltage terminal strip — often after a thermostat swap or DIY thermostat install.
Because the fix is a low-voltage thermostat-wire connection rather than any gas or high-voltage work, it is a reasonable homeowner check, provided the standard power-down precautions are followed first.
What You'll Notice
- The board shows four amber flashes, yet the furnace continues to heat and cool.
- On a call for cooling, the outdoor unit runs but the indoor blower does not, so little or no air comes from the registers.
- The evaporator coil may frost up because the AC runs without the indoor fan moving air across it.
- The problem often begins right after a thermostat replacement or rewire.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Disconnected or loose G wire at thermostat | Most common | ✓ DIY fix → |
How This Is Diagnosed
The check is simply confirming the G (fan) wire is landed and secure at both ends. At the thermostat, the green G conductor should be seated in the G terminal; at the furnace, the same wire should be secure on the board's G terminal. If the wire is present but loose, reseating it resolves the code; if the G conductor is missing entirely in the cable, new thermostat wire or a technician may be needed. Because both ends are low-voltage connections, this is a task many homeowners can perform safely.
How to Fix It: Reconnect the Thermostat G (Fan) Wire
What You'll Need
Steps
- Turn off power at the breaker or service switch AND shut off the gas supply Before opening anything, flip the furnace breaker (or the wall service switch) to OFF and turn the gas shutoff valve to OFF (handle perpendicular to the pipe). If you ever smell gas, leave immediately and call your gas company from outside.
- Check the G wire at the thermostat Gently pull the thermostat off its wall plate to expose the terminals. Find the G terminal and confirm a wire (typically green) is landed and tight. If it is loose, seat it fully and snug the terminal. If the G terminal is empty, note whether a spare wire is available in the bundle to use.
- Check the G terminal at the furnace control board Remove the furnace access panel and find the low-voltage terminal strip on the control board. Confirm the same G wire is secure on the board's G terminal, and reseat it if it is loose or disconnected.
- Reassemble Reattach the thermostat to its base and replace the furnace access panel. Make sure no wires are pinched.
- Restore gas and power Turn the gas valve back to ON (handle parallel to the pipe), then switch the breaker or service switch back to ON.
When to Call a Professional
Contact a licensed HVAC technician if:
- The G wire is missing from the cable and you are not comfortable running new thermostat wire.
- The G connections look correct at both ends but the code persists.
- You recently installed a new or smart thermostat and are unsure how the fan (G) terminal should be wired.
- Several thermostat wires appear loose, corroded, or damaged.
Frequently Asked Questions
Is it safe to keep using my furnace with a 4 amber flash code?
The furnace will still heat and cool, since York notes it operates normally with this code. The concern is that during cooling the indoor blower may not run, which can freeze the coil, so the G wire should be reconnected.
Which wire is the G wire?
The G terminal controls the indoor fan and is usually the green thermostat wire. This code means that signal is not reaching the board during a cooling call, so check the green wire at the thermostat and at the furnace board.
Why did this appear after I installed a new thermostat?
A loose or unconnected G wire is the most common cause, and it frequently happens during a thermostat swap. Confirming the G terminal is landed at both ends usually clears the code.
Sources
✓ Verified against manufacturer service manual — March 2026