York TM9T100C16MP11 Error Code 1 Red Flash: Flame Sensed Without Call for Heat
What Does Code 1 Red Flash Mean?
A 1 red flash code on the York TM9T100C16MP11 means the flame sensor is detecting a flame when the thermostat is not calling for heat and the gas valve should be closed. This is a serious safety concern. The control board has responded by turning on both the inducer motor and the supply air blower to ventilate any gas that may be present.
The most likely cause is a gas valve that is leaking internally or closing too slowly after a heating cycle ends. Gas valves contain internal seats and solenoids that can wear over time, allowing a small amount of gas to seep through even when the valve is supposed to be fully closed. This gas can reach the still-hot burner surfaces and ignite.
If you smell gas, evacuate your home immediately and call your gas utility company from outside. If you do not smell gas, turn off the furnace at the circuit breaker and contact a qualified HVAC technician immediately. Do not attempt to diagnose or repair gas valve issues yourself.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Leaking or slow-closing gas valve | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Immediately — a flame present without a call for heat indicates a potential gas valve leak that is a safety hazard
Sources
✓ Verified against manufacturer service manual — March 2026