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).
Honestly, about 70% of the Code 33 calls I go on don't actually need a new pressure switch. The switch is just reporting a problem that's upstream of it. Ignore this code and your furnace will keep locking out, and eventually trip into a hard lockout that won't auto-reset. Check the condensate drain and the rubber hose first before you order any parts.
Think of the pressure switch as a safety guard that makes sure the exhaust fan is actually pulling air before the gas turns on. If the switch doesn't click shut, the control board kills the ignition sequence to keep you safe. I always start by checking the simplest things, like the rubber tubing and the drain line, before even touching a multimeter. Nine times out of ten it's something free to fix.
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?
Most Carrier boards will try to reset themselves after a brief power cycle, but Code 33 can sometimes trigger a hard lockout if it fails too many times. Flip your furnace power switch off for a full minute to clear the board memory. Once you flip it back on, the inducer should kick in immediately to start a new trial for ignition.
Tools Required for Diagnosis
Multimeter (continuity/resistance setting)Wet/dry vacuumPhillips #2 screwdriverFlathead screwdriverFlashlightSmall wire brush or pipe cleaner for drain clearingBucket or towel for condensate overflowCompressed air can or small air compressorSafety glasses
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range0–2 ohms
ConditionIf Open (OL) or infinite, replace component.
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
Frequently Asked Questions
Can I bypass the pressure switch to get my Carrier furnace running?
Absolutely not, don't even think about it. I've seen rookies try this and it's incredibly dangerous. The pressure switch is the only thing standing between a safe furnace and a house full of carbon monoxide. If the switch isn't closing, there's a physical reason, like a blocked chimney or a dead inducer motor. Fix the actual problem, don't hide it. A CO detector won't save you if there's enough of it building up fast enough.
How do I clear a clogged condensate drain line on a Carrier furnace?
Start by finding the plastic trap inside the furnace cabinet. It usually looks like a black or clear U-shaped box near the bottom. Disconnect the hose at the bottom and drain it into a bucket first. If it's full of slime, that's your culprit right there. You can blow compressed air through the lines or use a shop vac to pull the blockage out from the exterior exit point. I also recommend pouring a cup of white vinegar through it afterward to kill the algae so it doesn't clog again in two weeks.
How do I know if the pressure switch itself is bad versus the hose or drain?
The suck test is the gold standard in the field. Disconnect the hose, put your meter on continuity, and suck on the port like a straw. If the switch clicks and closes, it's good. If it closes perfectly during that test but won't close when the furnace is actually running, your problem is a weak inducer, a clogged pressure port, or a drainage backup, not the switch itself. Don't buy a new switch until you've done this test. I've saved customers $40 a dozen times with this two-minute check.
What is the pressure switch part number for a Carrier 59TP6 furnace?
HK06WC090 is a very common one for Carrier, but here's the thing: these switches are calibrated to specific water column ratings. You can't just swap in any switch. Look at the label on your existing switch for a rating like -0.50" WC or -1.25" WC. If you use the wrong rating, the furnace will either never start or run under unsafe conditions. Always match that number exactly. Take a photo of the old switch label before you order anything.
Why does my Carrier furnace show Code 33 every fall when I first turn it on?
It's almost always the condensate drain. During summer, the water sitting in your furnace trap dries up and leaves behind crusty algae or a mineral film. When you first fire up the heat, that gunk gets pushed into the drain line and creates a plug. I tell all my customers to pour a little warm water and a splash of white vinegar through the trap every fall before heating season starts. Takes 5 minutes and it'll prevent this exact seasonal lockout year after year.