Carrier Furnace Lockout Issue
Carrier Furnace Shutting Off or Locking Out in NYC
If your Carrier furnace shuts down mid-cycle or won't stay running, codes 31 (pressure switch) and 33 (limit circuit) are the two documented codes behind that pattern -- and on older high-efficiency models, a well-known secondary heat exchanger issue is a common root cause.
What We Check First
Code 31 points to inadequate draft pressure -- often a restricted secondary heat exchanger or clogged condensate drain on older high-efficiency models.
Code 33 points to overheating or flame rollout -- most often restricted airflow from a dirty filter or blocked registers.
Quick Answer
A Carrier furnace shutting off or locking out mid-cycle is most often code 31 (pressure switch fault, often tied to a documented secondary heat exchanger corrosion issue -- Service Bulletin SMB 09-0022 -- or a clogged condensate drain) or code 33 (limit circuit fault from restricted airflow or flame rollout).
Common Causes
Secondary heat exchanger corrosion (code 31)
On older high-efficiency Carrier/Bryant/Payne furnaces, the secondary heat exchanger's polypropylene-laminated steel collector box can separate and corrode over time, restricting flue gas flow and causing the pressure switch to drop out. This was significant enough to be the subject of Carrier Service Bulletin SMB 09-0022.
Clogged condensate drain (code 31)
Condensing furnaces produce acidic condensate. If the drain trap or lines clog, water can back up into the inducer housing and block the pressure switch tubing.
Restricted airflow causing overheating (code 33)
By far the most common cause of a high-limit trip: a heavily soiled air filter, closed registers, or a blocked return grille causes the heat exchanger to overheat.
Flame rollout from a blocked heat exchanger (code 33)
If the secondary heat exchanger is badly restricted, the draft path chokes and the flame can roll backward out of the burner cell, tripping the manual-reset rollout switch.
Carrier Error Codes For This Issue
Codes below are informational — a code alone doesn't confirm the fix, and resetting power without addressing the underlying fault often just delays the problem.
31
What it means: Verified: pressure switch fault -- the switch did not close, or opened during a heating cycle, indicating inadequate draft pressure.
When service is needed: Service is needed to check the secondary heat exchanger, condensate drain, and inducer motor for restrictions causing inadequate draft.
33
What it means: Verified: limit circuit fault -- the high-limit switch or flame rollout switch opened due to overheating or restricted airflow.
When service is needed: Service is needed to check airflow (filter, registers, blower) and inspect for a restricted heat exchanger causing flame rollout.
DIY-Safe Checks vs. Call for Service
DIY-Safe
- Replacing a visibly dirty air filter and confirming supply registers and return grilles aren't blocked or closed.
- Resetting power once and observing whether the same code returns on the next call for heat.
Professional Required
- Inspecting the secondary heat exchanger and collector box for corrosion or restriction.
- Clearing the condensate drain and trap.
- Testing the inducer motor and pressure switch operation.
- Checking the blower motor and capacitor if airflow is weak.
FAQ
Why does my Carrier furnace keep shutting off?
Code 31 (pressure switch) or code 33 (limit circuit) are the two documented codes for this pattern. Code 31 often traces to a restricted secondary heat exchanger or clogged condensate drain; code 33 usually traces to restricted airflow.
Is the Carrier secondary heat exchanger issue a known problem?
Yes -- older high-efficiency Carrier/Bryant/Payne furnaces with a polypropylene-laminated secondary heat exchanger collector box were the subject of Service Bulletin SMB 09-0022 covering corrosion-related restriction.
Schedule Carrier Service
Need Carrier Repair in NYC?
A Carrier furnace shutting off or locking out mid-cycle is most often code 31 (pressure switch fault, often tied to a documented secondary heat exchanger corrosion issue -- Service Bulletin SMB 09-0022 -- or a clogged condensate drain) or code 33 (limit circuit fault from restricted airflow or flame rollout).