HVAC Code Lookup

Carrier Furnace: 3 Flashes

A Carrier furnace LED pattern starting with 3 flashes matches 4 documented codes: 31, 32, 33, 34. Watch the full pattern (short vs. long flashes) to tell them apart — all 4 are explained below.

Code 31: Pressure switch did not close, or reopened during the cycle

WarningShown as: 3 short flashes, 1 long flash

Pressure switch did not close, or reopened during the cycle. On single-stage furnaces this is the main vent pressure switch; on two-stage furnaces code 31 is the high-heat pressure switch or relay. If the switch stays open longer than 5 minutes the inducer shuts off for 15 minutes before retrying.

Manual wording: “31 PRESSURE SWITCH DID NOT CLOSE OR REOPENED - If open longer than five minutes, inducer shuts off for 15 minutes before retry.”

Documented for: 58SC0A, 58MCB (single-stage); 58MVP, 58UVB high-heat pressure switch (two-stage)

Likely causes, most common first

  1. Restricted vent, or the intake/exhaust pipe blocked by snow, ice, leaves or a nest
  2. Plugged condensate drain
  3. Excessive wind at the vent termination
  4. Disconnected or obstructed pressure tubing
  5. Defective inducer motor or low inducer voltage
  6. Defective or out-of-calibration pressure switch
  7. Improper vent sizing or inadequate combustion air supply

What you can safely check yourself

  • Go outside and check the furnace intake and exhaust pipes for snow, ice, leaves, or a bird/insect nest, and clear anything blocking them
  • Check the condensate drain line and trap for a clog or standing water
  • Check that the air filter is clean

When to call a technician

If the vent pipes are clear and the drain is not clogged, the inducer motor, pressure switch, or tubing needs a technician. Never jumper a pressure switch to make the furnace run.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 31

Code 32: Low-heat pressure switch did not close on a call for low heat, or opened during low heat

WarningShown as: 3 short flashes, 2 long flashes

Low-heat pressure switch did not close on a call for low heat, or opened during low heat. If it opens within 5 minutes after ignition, the next heating cycle is forced to high heat.

Manual wording: “32 LOW-HEAT PRESSURE SWITCH DID NOT CLOSE OR REOPENED - Indicates the low-heat pressure switch input failed to close on a call for low-heat, or opened during low-heat.”

Documented for: 58MVP, 58UVB and other variable-speed two-stage Carrier furnaces

Likely causes, most common first

  1. Improper vent sizing
  2. Plugged condensate drain
  3. Low inlet gas pressure (if a low gas pressure switch is used)
  4. Excessive wind
  5. Restricted combustion air supply
  6. Improper pressure switch wiring
  7. Failed or out-of-calibration pressure switch
  8. Water in the vent piping from a sagging pipe
  9. Disconnected or obstructed pressure tubing

What you can safely check yourself

  • Check outdoor intake and exhaust pipe terminations for snow, ice, or debris
  • Check the condensate drain and trap for a clog
  • Replace a dirty air filter

When to call a technician

Vent sizing, sagging vent pipe, and pressure switch calibration are all technician diagnostics. Do not bypass the switch.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Source

Full page for Carrier code 32

Code 33: Limit circuit fault

Soft lockoutShown as: 3 short flashes, 3 long flashes

Limit circuit fault. The high-limit switch, draft safeguard, flame rollout, or blocked-vent shutoff switch is open - the furnace is overheating or venting badly. The blower runs for 4 minutes or until the switch remakes. If the switch stays open more than 3 minutes the code changes to lockout 13.

Manual wording: “33 LIMIT CIRCUIT FAULT - Indicates the limit, or flame rollout is open. Blower will run for 4 minutes or until open switch remakes whichever is longer.”

Documented for: 58SC0A, 58SC0B, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Dirty filter or restricted duct system
  2. Loose blower wheel
  3. Defective switch or connections
  4. Defective blower motor or capacitor
  5. Inadequate combustion air supply (flame rollout switch open)
  6. Restricted vent or improper vent sizing
  7. Excessive wind
  8. Improper gas input adjustment

What you can safely check yourself

  • Check and replace the air filter - a clogged filter is the most common cause
  • Make sure supply registers and return air grilles are open and unblocked
  • Confirm the furnace door is fully seated
  • Check outdoor vent terminations for blockage

When to call a technician

If the code returns after a filter change, the limit or rollout switch is tripping for a reason a technician must diagnose. The flame rollout switch requires manual reset and must never be reset without finding out why it tripped.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 33

Code 34: Ignition proving failure

WarningShown as: 3 short flashes, 4 long flashes

Ignition proving failure. Flame was not sensed during the trial for ignition. The control will retry three more times before going to ignition lockout (code 14).

Manual wording: “34 IGNITION PROVING FAILURE - Control will try three more times before lockout #14 occurs.”

Documented for: 58SC0A, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Low inlet gas pressure
  2. Gas valve defective or turned off / manual shut-off valve closed
  3. Defective hot surface igniter
  4. Oxide buildup on the flame sensor
  5. Loss of control ground continuity - the green/yellow wire must be connected to the furnace sheet metal
  6. Flame sensor grounded or misplaced
  7. Inadequate flame carryover or rough ignition

What you can safely check yourself

  • Confirm other gas appliances in the house still work so you know gas service is on
  • Check that the furnace door is fully closed

When to call a technician

Cleaning the flame sensor, checking microamp flame current, and testing the igniter all require opening the burner compartment on a live gas appliance. Call a technician.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 34

Other Carrier codes