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 your York TM9T100C16MP11 furnace indicates that the control board is receiving a "Y" signal (call for cooling) from the thermostat without a corresponding "G" signal (blower fan). In a properly wired system, when the thermostat calls for cooling, it should send both the Y signal to activate the air conditioner or heat pump and the G signal to turn on the indoor blower fan.
The good news is that this code does not prevent your furnace from operating. Both heating and cooling functions will continue to work normally despite the wiring issue. The control board displays this code as an alert so that the wiring problem can be corrected, ensuring proper system operation and efficiency.
The most common cause is a loose, disconnected, or missing "G" wire at the thermostat or at the furnace control board terminal strip. This can happen during thermostat installation or replacement, or if a wire connection has come loose over time.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Disconnected or loose G wire at thermostat | Most common | ✓ DIY fix → |
How to Fix It: Check Thermostat Wiring
What You'll Need
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the circuit breaker for your furnace in your electrical panel and flip it to OFF. 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 thermostat wiring Remove the thermostat cover from the wall plate to expose the wiring terminals. Look for the "G" terminal and verify that a wire (typically green) is connected securely. If the wire is present but loose, push it firmly into the terminal and tighten any set screw. If the G wire is missing entirely, the thermostat may need to be rewired.
- Check the furnace control board terminals At the furnace, remove the access panel and locate the low-voltage terminal strip on the control board. Verify that the "G" wire is connected securely to the G terminal. If it is loose or disconnected, reconnect it firmly.
- Restore gas supply and electrical power Replace the furnace access panel and thermostat cover. Turn the gas supply valve back to the ON position (parallel to the pipe), then flip the furnace circuit breaker back to ON.
When to Call a Professional
Contact a licensed HVAC technician if:
- The G wire is missing and you are not comfortable running new thermostat wire
- The wiring appears correct but the code persists
- You recently installed a new thermostat and are unsure about the wiring
- Multiple thermostat wires appear loose or damaged
Sources
✓ Verified against manufacturer service manual — March 2026