Carrier Furnace 3 Short 3 Long Flashes: Code 33 Pressure Switch Fault

Quick Answer

Carrier furnace 3 short 3 long LED blinks is Code 33, a pressure switch fault. The pressure switch monitors draft pressure from the inducer motor and prevents the gas valve from opening until safe pressure is confirmed. The most likely fix is clearing a clogged condensate drain line (free, 15 minutes). Check the drain first before replacing the switch. If the drain is clear, inspect the rubber hose from the inducer to the switch for kinks or cracks, then test the switch itself with a multimeter. Replacing the switch costs $20-45 in parts.

CarrierFurnaceSeverity: highDifficulty: intermediate85% DIY Success

Time to Fix

20–75 min

Difficulty

intermediate

Parts Cost

$3 – $45

Tools Needed

Multimeter (continuity/resistance setting), Wet/dry vacuum

Wrong code?

Not seeing 3 Short 3 Long on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

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HUB

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pressure switch fault

GoodmanF02

What Does the 3 Short 3 Long Code Mean?

The pressure switch is not detecting proper negative pressure from the inducer motor. Most commonly caused by a clogged condensate drain, not a bad switch.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Condensate drain line clogged with algae or debris40%

Rubber pressure hose kinked, cracked, or disconnected24%

Failed pressure switch diaphragm14%

Blocked flue or vent pipe restricting exhaust12%

Weak or failing inducer motor not generating enough draft10%

Symptoms You May Notice

Furnace starts, inducer motor runs, then shuts down before ignition
Control board LED shows 3 short blinks followed by 3 long blinks
No clicking from gas valve during heating cycle
Furnace restarts repeatedly but never reaches ignition stage
Water pooling near the furnace base (condensate backup)

Can you reset a Carrier furnace to clear the 3 Short 3 Long code?

After resolving the root cause of Code 33, reset the furnace by setting the thermostat to OFF, waiting 30 seconds, then back to HEAT. If the code returns, cycle the circuit breaker off for 30 seconds. The code will clear automatically after one successful heating cycle completes without the fault recurring.

Tools Required for Diagnosis

Multimeter (continuity/resistance setting)Wet/dry vacuumFlathead screwdriverFlashlightSmall wire brush or pipe cleaner for drain clearing

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Locate the condensate drain line exiting the furnace cabinet

Locate the condensate drain line exiting the furnace cabinet. It is typically a white PVC or clear vinyl tube running to a floor drain or condensate pump. Disconnect the drain at the trap or the first accessible fitting and check for blockage. Use a wet/dry vacuum at the drain outlet to clear the line.

Locate the small rubber hose (typically 3/16" to 1/4" diameter)

With power off

With the furnace running (do not touch internal components while

Inspect the flue vent pipe from the furnace outlet to the

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
Carrier Pressure SwitchHK06WC090 · $20–$45	HK06WC090	$20 – $45
Condensate Drain Kit with TrapN/A · $8–$20	N/A	$8 – $20
Pressure Switch HoseN/A · $3–$8	N/A	$3 – $8

Verify part fitment for your Carrier model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $24 (Parts) vs $2750 (New Machine)

Frequently Asked Questions

Can I bypass the pressure switch to get my Carrier furnace running?

No. The pressure switch is a safety device that confirms the inducer motor is creating adequate draft before gas is allowed to flow. Bypassing it can allow gas to accumulate in an improperly vented combustion chamber, which is a fire and carbon monoxide hazard. Always fix the root cause rather than bypassing the switch.

How do I clear a clogged condensate drain line on a Carrier furnace?

Locate the condensate trap or drain tube exiting the furnace. Disconnect it at the lowest accessible point. Use a wet/dry vacuum at the drain outlet end to pull the blockage clear. You can also flush the line with a mixture of distilled white vinegar and water (50/50) to dissolve algae buildup. Run the vacuum for 30-60 seconds to ensure the full line is clear.

How do I know if the pressure switch itself is bad versus the hose or drain?

Disconnect the rubber hose from the pressure switch and apply gentle suction directly to the switch port with your mouth while testing continuity across the terminals with a multimeter. If the switch closes (shows continuity) under suction, the switch is working and the problem is upstream: check the hose, condensate drain, or inducer motor. If the switch never closes under suction, it has a failed diaphragm and needs replacement.

What is the pressure switch part number for a Carrier 59TP6 furnace?

The most common pressure switch for the 59TP6 series is Carrier part HK06WC090, but exact part numbers vary by model suffix and BTU rating. Always check the model and serial number label inside the cabinet door and verify using the Carrier parts lookup at carrier.com or an HVAC supply house.

Why does my Carrier furnace show Code 33 every fall when I first turn it on?

Algae and mold grow inside the condensate drain during summer when the furnace is idle. The first heating cycle of the season pushes condensate through a partially blocked line, triggering Code 33. Annual maintenance before heating season should include flushing the condensate drain with a vinegar solution and vacuuming the trap. This prevents the recurring seasonal Code 33.

Related Carrier Furnace Error Codes

Code: HUBCarrier Furnace LED Flash Codes: Complete Fault Code Guide Code: 13Carrier Furnace Code 13: Limit Device Fault Lockout Code: 34Carrier Furnace Code 34: Ignition Proving Failure Code: RESETHow to Reset a Carrier Furnace and Clear LED Fault Codes

Same Fix on Other Brands

Goodman furnaceF02: Goodman Furnace F02: Pressure Switch Stuck Closed

Models Known to Experience 3 Short 3 Long Errors

This repair applies to most Carrier furnaces with this error code. Common model numbers include:

59TP6, 59MN7, 59SP5, 59TP6A, 58PAV

Last verified for technical accuracy on March 14, 2026