33 Limit circuit fault

What the control is telling you

The limit or a flame rollout switch is open, or the furnace is in high-heat-only mode after 2 successive low-heat limit trips. Blower runs at least 4 minutes; if open over 3 minutes the code becomes lockout 13.

Applies to: 59TP6A two-stage variable-speed ECM multipoise condensing gas furnace, all sizes (040-10 to 120-22)

Likely causes

• Loose blower wheel
• Defective switch or connections
• Improper low- or high-heat gas input
• Wrong limit switch or no limit gasket
• Dirty filter or restricted ducts
• Open flame rollout switch (manual reset)

Test and repair steps

1. Warning: Shut off 115-V power and gas, and lock/tag out before working inside the furnace; there may be more than one power source.
2. Replace dirty filters and check supply/return ducts for restrictions.
3. Check the blower wheel is secure.
4. Check gas input by clocking the meter and manifold pressure in both stages.
5. Inspect the limit switch, its gasket and wiring; replace if defective or incorrect.
6. If the rollout switch is open, correct combustion air, gas pressure, burner/orifice alignment or venting before manually resetting it.
7. Verify temperature rise is within the rating-plate range after repair.

Settings for these units

• Filter sizes by casing width and filter cabinet · 59TP6A
• Minimum clearances to combustibles · 59TP6A