Lennox furnace Alert code 200: Rollout limit switch open
Alert 200 means the furnace rollout limit switch is open. Lennox rates this Service Urgent, and its own guidance is to correct the cause of the rollout trip before anything else.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide. Non-communicating Lennox furnaces use a different diagnostic method.
What it means
A rollout switch opens when flame escapes forward out of the burner compartment instead of being drawn through the heat exchanger. Lennox classifies this as Service Urgent, the highest priority in its scheme, and the wording of its guidance is worth noting: it says to correct the cause of the rollout trip, then reset the switch, then test operation. That ordering is deliberate. Resetting a rollout switch without establishing why it tripped puts the furnace straight back into the condition that caused it, and the conditions that cause rollout are the ones that produce carbon monoxide.
Most common causes
- A blocked or obstructed vent pipe, either intake or exhaust, which Lennox lists explicitly.
- Unstable flame, which Lennox directs you to investigate for its own cause.
- A blocked or restricted heat exchanger.
- A failed or weak combustion air inducer.
- Dirty or misaligned burners.
- Insufficient combustion air in a confined furnace space.
- Overfiring, with gas pressure set too high.
How to diagnose it
- Treat this as a stop-and-call fault. Lennox rates it Service Urgent and its causes are dominated by conditions that produce carbon monoxide.
- Check that carbon monoxide alarms are present and working. If any has sounded, leave and call the fire service from outside.
- Check the outside vent termination for obstruction, since Lennox names intake and exhaust blockage first.
- Note that the alert clears automatically once the rollout switch closes, which means the alert disappearing does not mean the cause was found.
How to fix it
- Turn the furnace off and call a licensed heating technician.
- Do not reset the rollout switch to restore heat. Lennox's own sequence puts correcting the cause before resetting the switch.
- Clearing an obstructed vent termination is within homeowner reach and addresses the cause Lennox lists first.
- Have working carbon monoxide alarms on every level of the home and outside each separate sleeping area, which is what NFPA 720 and US Consumer Product Safety Commission guidance call for. That includes the level the furnace is on — basements, attics and utility spaces are where a combustion fault starts, and an alarm only on bedroom floors can miss it for a long time. This fault is one of the situations those alarms exist for.
- Flame stability, heat exchanger, inducer, and burner work are all licensed tasks.
Notes
Lennox documents that this alert clears automatically once the switch closes. That is worth understanding: an alert that has gone away is not evidence the problem is solved, and on a rollout fault that distinction matters more than on any other code.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
The switch itself sits in the repair guide's $150–350 limit-and-rollout band, and the guide's note on that line — find why it tripped, not just the part — is the entire economics of this alert. A rollout switch that tripped is usually a working switch reporting a real event, and the bill depends on the event: a blocked flue or vent cleared, a combustion air problem corrected, or — the finding that changes everything — heat exchanger damage, which at $1,000–2,500+ is usually the point where the money argues for replacing the furnace rather than repairing it. A quote that only replaces the switch without naming what made flame leave the burner compartment is half a diagnosis at full price.
DIY or call a pro
This is a no-DIY page, and the reason is in what the switch witnessed: flame physically escaping the burner compartment, which is the precondition for both fire and carbon monoxide. There is no filter to change or termination to clear that safely explains a rollout trip, and the reset button on the switch is the one control a homeowner must leave alone — Lennox's documented order is cause first, reset second, and reversing it re-runs the event that tripped the protection. If the furnace is locked out on this alert, leave it off and book the visit as urgent, which matches Lennox's own Service Urgent rating. Mention any recent vent work, chimney work or cabinet damage when you call; those are the histories that shorten this particular diagnosis.
The same fault on other brands
Rollout protection is universal but the reporting is not: Allied boards flash 8, Goodman's legacy boards flash 6, and the shared Goodman/York table uses 5 red flashes (which also covers an open fuse, an ambiguity Lennox avoids). Carrier does not give rollout its own code at all — it folds into the limit circuit code 33. The rollout switch hub maps the full set.
What this looks like
From the outside, this usually shows up as the furnace heats then trips out on high limit — that walkthrough covers what to check in order, and when it stops being a homeowner job.