Trane furnace 6 flashes: 115 volt AC power reversed, or poor grounding
Six flashes means the line voltage supply has line and neutral reversed, or the furnace is not properly grounded. It is a wiring fault rather than a component failure.
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
Flame detection works by rectification, which passes a very small current through the flame to ground. That only works if the supply is connected the right way round and the furnace has a solid ground path, so the control checks both before it will operate. This code means the check failed. It is worth recognising as a wiring fault rather than a broken part, because it almost always follows electrical work or a new installation. A furnace that ran for years and suddenly shows this usually means something in its supply circuit has been changed or has come loose.
Most common causes
- Line and neutral reversed at the furnace, at a junction box, or at the outlet feeding it.
- A missing or disconnected ground wire at the furnace.
- A loose neutral connection in the circuit.
- A recently replaced outlet or newly run circuit wired incorrectly.
- A furnace that has been moved, replaced, or reconnected.
- A control board that has failed in a way that misreads the supply.
How to diagnose it
- Establish whether any electrical work has been done recently — in the house generally, not only at the furnace.
- If the furnace is newly installed or reconnected, treat this as an installation fault rather than a developed failure.
- Note whether other faults have appeared alongside it. Reversed polarity and poor grounding also produce weak flame signal and igniter codes, so a cluster usually has one cause.
- Do not test the supply yourself unless you are qualified to work at a live panel.
How to fix it
- This is electrical work on the furnace supply and belongs to a licensed electrician or heating technician.
- If the furnace was just installed or reconnected, call whoever did the work — this is a wiring error, not a developed fault.
- Have the ground path checked as well as the polarity, since this code covers both.
- Do not attempt to bypass the check. The control is refusing to run because it cannot trust its own flame detection.
Notes
Because the same conditions break flame sensing, a furnace showing this alongside a history of ignition faults usually has one electrical cause behind all of them. Fixing the supply often resolves several apparently unrelated codes at once.