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.