Flashing LED diagnostic code sequence indicating a rollout switch trip on a Carrier furnace
Short answer
A flame rollout switch trip on a Carrier furnace displays as Flash Code 33 with three short and three long flashes, or Code 13 for rollout lockout. This critical safety fault indicates combustion flames have spilled outside the burner compartment. Never manually reset the tripped switch before inspecting the heat exchanger for cracks, clearing flue blockages, and confirming manifold gas pressure reads 3.5 inches water column for natural gas.
On a Carrier furnace, a flame rollout switch trip registers as Flash Code 33 (three short flashes followed by three long flashes) or Code 13 (one short flash followed by three long flashes for rollout lockout).
A flame rollout switch trip indicates that combustion flames or flue gases have spilled outside the burner vestibule rather than pulling cleanly into the heat exchanger cells. According to industry combustion safety standards, rollout events present immediate fire and carbon monoxide hazards, requiring an immediate shutdown and diagnostic inspection.
If you only do one thing: Never manually reset a tripped rollout switch before inspecting the heat exchanger for cracks, checking manifold gas pressure at 3.5 in. w.c. for natural gas, and verifying flue draft.
- Decode the LED Sequence: Carrier control boards display two-digit fault codes where short flashes represent the tens digit and long flashes represent the units digit, presenting Code 33 (3 short, 3 long) as an active open circuit or Code 13 (1 short, 3 long) as a manual-reset lockout.
- Isolate Power and Test Continuity: Disconnect 120-volt line power, access the burner compartment, and verify open continuity across the thermal rollout switch terminals using a digital multimeter set to ohms; an open circuit confirms the switch has tripped past its calibrated limit (typically 300°F to 350°F).
- Inspect the Heat Exchanger: Perform a visual inspection and combustion analysis (measuring flue carbon monoxide below 100 ppm air-free) to detect cell breaches, plugged secondary condensing coils, or blocked flue passages causing backpressure.
- Verify Draft Inducer and Venting: Measure static draft pressure using a digital manometer to ensure the inducer motor draws sufficient negative pressure (typically between -0.40 and -0.90 in. w.c., per furnace data plate) through the vent system.
- Check Gas Manifold Pressure: Connect a manometer to the gas valve outlet port during ignition to ensure pressure matches specification—3.5 in. w.c. for natural gas or 10.0 to 11.0 in. w.c. for liquid propane—preventing burner overfiring.
- Watch out for: Bypassing or repeatedly pressing the manual reset button without resolving the root cause, which can melt internal wiring harnesses or crack the furnace cabinet within 2 to 5 minutes of operation.
- Watch out for: Failing to inspect the secondary heat exchanger on high-efficiency Carrier 90%+ AFUE condensing furnaces, where internal polypropylene plugging frequently causes rollout trips.
- Watch out for: Replacing the switch with an incorrect thermal rating; always match the exact OEM cutoff rating stamped on the bracket (such as 300°F or 350°F).
If diagnostic testing confirms heat exchanger failure or persistent draft blockages, contact ARS/Rescue Rooter at 866-399-2885 for professional heating system repair backed by the Exceptional Service Guarantee.