York TM9T100C16MP11 Error Code 4 Amber Flashes: Y Signal Without G Signal
What Does Code 4 Amber Flashes Mean?
A 4 amber flashes code on the York TM9T100C16MP11 means the control board is receiving a "Y" signal (a call for cooling) without the matching "G" signal that should turn on the indoor blower. In a correctly wired system, a cooling call sends both Y and G together. The board flags the missing G as an alert.
The good news is this code does not stop the furnace from working — York states the unit will operate normally in both heating and cooling. It is displayed so the wiring can be corrected, since without the G signal the blower may not run in fan-only mode and airflow behavior during cooling can be affected.
The most common cause is a loose, disconnected, or missing green G wire at the thermostat or at the furnace control board terminal. This often happens during thermostat installation or replacement, or when a connection works loose over time. Because it is a low-voltage (24-volt) thermostat wiring alert, it is distinct from the 9 red flashes code, which is a 120-volt line-voltage polarity or grounding problem.
What You'll Notice
- The board shows four amber flashes while heating and cooling still function normally.
- The green (G) fan wire may be loose or disconnected at the thermostat or the furnace.
- The blower may not run when the thermostat fan is set to "On".
- There are no other symptoms — the furnace otherwise operates as expected.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Disconnected or loose G wire at thermostat | Most common | ✓ DIY fix → |
How to Fix It: Check and Reseat the Thermostat G (Fan) Wire
What You'll Need
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF before opening any panel. Also turn the gas supply valve to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave your home immediately and call your gas company from outside.
- Check the G wire at the thermostat Gently pull the thermostat off its wall base to expose the terminals. Find the "G" terminal and confirm a wire (usually green) is seated securely. If it is loose, push it firmly into the terminal and tighten any set screw. If the G terminal is empty, the wire may be disconnected further back.
- Check the G wire at the furnace control board At the furnace, remove the access panel and locate the low-voltage terminal strip on the control board. Confirm the "G" wire is connected securely to the G terminal and reseat it if it is loose or disconnected.
- Reassemble and restore power and gas Replace the furnace access panel and reattach the thermostat to its base. Turn the gas supply valve back to ON (parallel to the pipe), then flip the furnace breaker back to ON.
When to Call a Professional
Contact a licensed HVAC technician if:
- The 4 amber flash code remains after reseating the G wire at both the thermostat and the control board
- The blower will not run in fan-only ("On") mode
- The G wire is missing entirely and new thermostat wire is needed
- You are unsure which terminal the green wire belongs to, or the thermostat has unfamiliar wiring
Frequently Asked Questions
Is a 4 amber flashes code serious on my York TM9T?
No. The furnace still heats and cools normally. It is a low-voltage wiring alert about the missing fan (G) signal that is worth correcting.
What is the G wire?
The G wire is the green fan wire that tells the furnace to run the indoor blower, such as during cooling or fan-only mode.
Will this code stop my heat from working?
No. York states both heating and cooling continue to operate; only the fan-signal wiring is being flagged.
Sources
✓ Verified against manufacturer service manual — March 2026