33.1 Main limit circuit switch open

What the control is telling you

A switch in the main limit circuit (auto-reset limit or a manual-reset rollout switch) has opened.

Applies to: Carrier 59SC6A

Likely causes

• Restricted airflow (filter, ducts, registers)
• Airflow setting too low / high temperature rise
• Blower failure
• Over-firing
• Flame rollout from combustion air, gas pressure, burner or vent problem

Test and repair steps

1. Warning: Never jumper the limit or flame rollout switches.
2. Check and clean/replace the filter (it is external to the casing); open all registers and check ducts for restrictions.
3. Measure temperature rise with the blower door on: duct thermometers in supply and return near the furnace, out of line-of-sight of the heat exchanger. It must be within the rating-plate range; midpoint or slightly above is recommended.
4. If the rise is high, check gas input rate, altitude derate, and duct static; static above 0.50 in. w.c. is excessive.
5. Raise the Ht heating airflow setting to lower the rise (lower it to raise the rise).
6. Check manifold pressure for 40,000-120,000 BTUh models it must be 2.8-3.8 in. w.c. on natural gas; on the 26,000 BTUh model (green label on gas valve solenoid, Table 26) do not exceed 1.8 in. w.c.
7. Check blower operation with the Ct component test; check the blower wheel and motor are clean.
8. If a manual-reset flame rollout switch tripped, correct combustion air, gas pressure, burner/orifice position or venting before resetting it.
9. Verify the limit works run the furnace at least 5 minutes, slowly block the return air until the limit trips, then unblock; burners should relight once it cools.

Settings for these units

• Filter sizes by casing width and filter cabinet · 59SC6A
• Minimum clearances to combustibles · 59SC6A