Lennox furnace Alert code 349: O to R jumper cut on the furnace control
Alert 349 means the O to R configuration link on the furnace control has been cut. Lennox warns it can cause a brown out and low voltage, and the fix is to restore the link by hard-wiring R to O at the terminal strip.
Applies to: Lennox gas furnaces running in non-communicating mode. Lennox states this alert is only applicable in non-communicating mode.
What it means
The O terminal is the reversing valve signal used with a heat pump. On a furnace configured for non-communicating operation, Lennox uses a factory link between R and O that has to be present unless the installation specifically requires it cut. When it has been cut and not restored, the control reports 349. The reason this matters more than a configuration nag is what Lennox says it can cause: a brown out, and low voltage that will generate further alert codes of its own. So a furnace showing 349 alongside voltage alerts is not necessarily suffering two faults — the second may be downstream of the first, and chasing the voltage codes first would be chasing a symptom.
Most common causes
- The R to O link cut during installation for a heat pump configuration and never restored when the configuration changed.
- The link cut in error during setup.
- A control board replaced without the link being reinstated to match the installation.
- A system converted from dual fuel to gas-only without the configuration being reverted.
How to diagnose it
- Note whether the system is running in non-communicating mode — Lennox says the alert applies only there.
- Check the fault history for low voltage alerts alongside this one; Lennox links the two directly.
- Note whether the system was recently converted, reconfigured, or had a control board replaced.
- The terminal strip is inside a live cabinet, so the check itself is technician work.
How to fix it
- Restore the configuration link by hard-wiring the R and O terminals on the terminal strip, which is Lennox's stated remedy.
- Resolve this before investigating any accompanying voltage alerts, since Lennox says this fault can generate them.
- If the installation genuinely requires the link cut, the configuration and the mode need to be made consistent rather than the alert dismissed.
Notes
This is a configuration fault rather than a failure, which makes it one of the more satisfying codes on the list — nothing is broken, and the remedy is a wire.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Restoring a cut configuration link is minutes of work on the control's low-voltage terminals, so the honest price is a standard service call at $75–150 — and the savings this page can offer is in what you avoid paying for. Lennox warns that a cut O-to-R link can produce a brown-out and low voltage, which means it can generate voltage alerts that look like their own faults; a technician chasing those without spotting the 349 behind them can burn a visit (or quote a transformer) treating symptoms. If the link was cut during a heat pump installation or removal — the legitimate reason it ever gets cut — and the configuration no longer matches the equipment, the correction belongs on the tab of whoever did that installation. Nothing about this alert justifies a parts bill until the link question is answered.
DIY or call a pro
The fix itself is technician work — it means confirming whether this installation is supposed to have the link (heat pump systems legitimately run without it) and then working on the control's terminal strip, and guessing wrong in either direction misconfigures the furnace. The homeowner's role is historical testimony, and on this alert it is unusually decisive: has a heat pump ever been added, replaced or removed? Did the alert begin after any equipment change or service visit? A 349 that appears out of nowhere on an untouched system is odd and worth saying so; one that follows equipment work almost names its own culprit. Also useful: report every voltage-related alert in the history alongside the 349, in order if possible — presenting them as a package invites the correct diagnosis (one cause, several symptoms) instead of piecemeal fixes for each code separately.
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.