Trane furnace 9 flashes: Ignitor circuit problem, or low voltage between neutral and 24 volt common
Nine flashes directs you to check the ignitor circuit and the voltage between line neutral and 24 volt common, which Trane specifies should be under 2 volts. A higher reading indicates a grounding problem.
Applies to: Trane and American Standard upflow, downflow and horizontal gas-fired direct vent condensing furnaces covered by Installer's Guide 18-CD29D1-14-EN, using the integrated furnace control described there.
What it means
Trane's wording for this code is unusually specific and worth reading closely. It names two checks: the ignitor circuit, and the voltage between line neutral and the 24 volt common, with a stated threshold of less than 2 volts. That second check is a grounding test. If neutral and 24 volt common differ by more than a couple of volts, the furnace's ground reference is not what the control expects, and flame rectification — which depends on a clean ground path — becomes unreliable. So while the code names the ignitor first, Trane's own parenthetical is explicit that this is a possible grounding problem.
Most common causes
- A grounding problem, indicated by more than 2 volts between line neutral and 24 volt common.
- A missing or loose ground connection at the furnace chassis.
- A loose blower motor or low voltage common ground.
- A failed or shorted hot surface ignitor.
- Damaged ignitor wiring shorting to the cabinet.
- A failed ignitor relay on the control board.
- A neutral problem in the house wiring producing the voltage offset.
How to diagnose it
- Note whether the ignitor glows at all during a start attempt, if the furnace has a sight glass.
- Mention recent electrical work, control board replacement, or blower motor replacement to the technician — each disturbs a ground this fault depends on.
- Note whether other faults have appeared alongside it. Grounding problems produce several codes at once, and a cluster is itself a clue.
- Do not attempt the voltage measurement yourself unless qualified. It is taken at live terminals.
How to fix it
- This is technician work, involving the ignition circuit and live voltage measurements.
- Ask the technician to make the neutral to 24 volt common measurement Trane specifies before replacing the ignitor. A grounding fault that goes unfound will simply reappear.
- Have ground connections to the furnace chassis checked and tightened, particularly the blower motor and low voltage common grounds.
- If the offset traces back to house wiring rather than the furnace, that needs an electrician.
Notes
Trane publishing a numeric threshold makes this one of the more objectively testable furnace faults. A reading above 2 volts is a grounding problem by the manufacturer's own definition rather than by judgement.
Related
- 6 flashes — 115 volt power reversed or poor grounding
- Continuous on — Replace the integrated furnace control