What This Error Means

Error code 33 means a limit, draft safeguard, flame rollout, or blocked-vent switch is open, or that the furnace is operating in a restricted mode after repeated low-heat limit trips.

Symptoms You May Notice

Burners shut off while the blower continues; furnace short cycles; code 33 flashes; weak airflow; high-heat-only operation on applicable models; code may change to 13 if the safety remains open long enough.

Common Causes

Dirty filter; restricted airflow; closed registers; blower speed or motor problem; excessive gas input; dirty evaporator coil; open draft safeguard; venting restriction; flame rollout; damaged safety-circuit wiring; failed limit switch.

Safe Troubleshooting Steps

1. Turn off furnace power. 2. Replace a dirty filter and open all supply and return vents. 3. Confirm furniture is not blocking return grilles. 4. Check accessible vent terminations for obvious obstruction. 5. Do not reset a rollout switch or bypass any safety. 6. If the code clears, observe one cycle only. 7. A technician should identify the open safety and test airflow, temperature rise, gas input, venting, combustion, blower performance, and wiring.

Parts Commonly Involved

Main limit switch; draft safeguard; flame rollout switch; blocked-vent switch where used; blower motor and wheel; duct system; integrated furnace control.

When to Call a Technician

Call a technician if code 33 returns after a filter and vent check, if airflow is weak, or immediately if a rollout or blocked-vent safety is open.

Safety Notice

Never bypass any safety switch. A rollout or blocked-vent fault can indicate a serious combustion hazard requiring immediate professional diagnosis.

Manufacturer Documentation