York TG9S Furnace Problems & Error Codes — Overview
The York TG9S is a residential gas furnace that uses LED flash codes on the control board to signal faults. The most frequent issue techs see is fault code 9, shown as a rapid red flash, which points to reversed line-voltage polarity or a grounding problem at the unit. Other common codes include 7 red flashes for ignition lockout, 8 red for flame loss, and 11 red for limit-switch trips. The control board sits in the lower compartment with a sight glass for reading the LED during operation. Power loss clears the stored code, so always check the flash pattern before cycling the disconnect.
Most Common Problems on This Model
- Rapid red flash (fault 9) locks out heating Reversed L1/L2 polarity at the furnace supply or missing equipment ground causes the control to lock out, so verify incoming line-voltage polarity at the disconnect and check chassis ground continuity to the branch circuit before replacing the board.
- Rapid red flash on twin furnace setups Incorrect 24V phasing between twinned furnaces or reversed R and C at the transformer secondary triggers fault 9, so verify twinning harness wiring and 24V phasing match York instructions.
- 7 red flashes, ignition lockout after no flame Failed hot-surface igniter, closed gas valve, or dirty flame sensor prevents ignition, so inspect the igniter for cracks, confirm gas flow at the valve, and clean or replace the flame sensor rod.
- 8 red flashes, flame loss during heating Low gas pressure, dirty flame sensor, or faulty gas valve causes repeated flame loss, so check inlet gas pressure, clean the flame sensor, and test the gas valve for correct operation.
- 11 red flashes, limit switch open Failed blower motor, blocked filter, or restricted blower wheel stops airflow and trips the limit, so replace the blower motor or wheel and check the filter and return plenum.
- 12 red flashes, open igniter circuit Cracked hot-surface igniter or loose igniter wire connection opens the circuit, so inspect the igniter element for cracks and check wire terminals at the igniter and control board.
- 4 amber flashes, improper thermostat wiring Control receives Y (cooling call) without G (fan), so verify thermostat wiring at the board and correct any miswired Y or G terminals.
- Steady red LED, control failure Faulty ignition control board fails internal diagnostics, so replace the control board after ruling out reversed polarity and shorted flame-sensor leads.
Parts That Commonly Fail
| Part | Notes |
|---|---|
| Ignition control board | Amazon | Fails after polarity faults or internal component wear, always verify wiring and grounding before replacement |
| Hot-surface igniter | Amazon | Cracks from thermal cycling or rough handling, visible hairline cracks require replacement |
| Flame sensor rod | Amazon | Carbon buildup on the sensor tip causes weak signal and flame-loss codes, clean with fine abrasive or replace |
| Blower motor | Amazon | Bearings fail or capacitor weakens, causes limit trips and reduced airflow |
When to Call a Pro
Call a licensed HVAC technician for any gas-supply work, including gas-valve replacement, pressure testing, or combustion analysis. Line-voltage polarity checks and control-board diagnostics require proper metering and familiarity with York fault-code logic. Cracked heat exchangers or recurring limit trips need professional inspection to rule out blocked venting or unsafe combustion conditions. If you see fault 9 and basic polarity checks do not clear it, a tech with twinning or grounding-fault experience should trace the circuit.