Carrier Error Code 33: Limit/Rollout Switch Lockout

Severity
Critical
Type
led blink pattern
Where to look
sight glass
Estimated repair cost: $75 — $200 (parts + labor)
Warning: Your Carrier furnace has detected a limit or rollout switch lockout, indicating a potential safety issue. Shut down the furnace immediately and call a qualified technician.

LED Blink Pattern

3 short blinks, 3 long blinks.

How to Fix Carrier Code 33

  1. Step 1: Turn off the furnace at the thermostat.
  2. Step 2: Check and replace the air filter if it's dirty.
  3. Step 3: Ensure that all vents (both intake and exhaust) are free from obstructions like snow, ice, or debris.
  4. Step 4: After waiting a few minutes, turn the furnace back on at the thermostat. If the error reappears, call a qualified HVAC technician immediately.

How to Reset Carrier Code 33

  1. Turn off the furnace power switch, usually located on or near the unit.
  2. Turn off the circuit breaker for the furnace. Wait 30 seconds.
  3. Restore power at the breaker and flip the furnace switch back on.
  4. Set the thermostat to call for heat. If the error code returns after 2-3 ignition attempts, the fault requires repair.

Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.

What Does Carrier Error Code 33 Mean?

Error code 33 on your Carrier furnace signifies that the unit's control board has detected an open limit switch or rollout switch for longer than is considered safe. These switches are safety devices designed to shut down the furnace if it overheats or if flames are detected where they shouldn't be (like outside the burner box). The control board locks out after detecting this condition to prevent further operation until the issue is resolved. This lockout requires a manual reset, usually by cycling power to the furnace. This error is often triggered by a genuine safety concern, such as a blocked vent, a malfunctioning fan, or a cracked heat exchanger. It's crucial to address the underlying cause rather than simply resetting the furnace, as repeated lockouts could indicate a dangerous condition. Ignoring this error can lead to equipment damage, fire, or carbon monoxide poisoning.

Common Causes

  • Blocked flue or vent pipe restricting exhaust flow
  • Dirty air filter restricting airflow across the heat exchanger
  • Faulty blower motor or capacitor causing insufficient airflow
  • Cracked heat exchanger allowing flame rollout
  • Defective limit switch or rollout switch

Diagnostic Steps

If... Check Expected
Furnace is locked out and displaying error code 33 Visually inspect the flue pipe for any obstructions or disconnections. Flue pipe is clear, properly connected, and has proper slope.
Flue appears clear Measure the resistance across each limit switch and rollout switch with a multimeter. Each switch should read close to 0 ohms (closed circuit) when the furnace is cool. High resistance indicates an open switch.
A limit switch is open Check the temperature rise across the heat exchanger using a thermometer at the return and supply ducts. Temperature rise should be within the range specified on the furnace's rating plate (typically 40-70°F).

Applicable Carrier Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Limit Switch KH42WZ008 — Find Part
Rollout Switch LH33WZ557 — Find Part
Blower Motor Capacitor — — —

Related Codes on Same Models

Other codes that affect the same Carrier model families as 33.

Code Description Severity
12 Blower On After Power Up Warning
13 Limit/Rollout Fault Critical
14 Ignition Lockout Critical
21 Gas Heating Lockout Critical
22 Abnormal Flame Critical
23 Pressure Switch Fault Warning
24 Secondary Voltage Fuse Warning
31 High Stage Pressure Fault Warning
32 Low Flame Sense Warning
34 Ignition/Flame Sense Fault Warning