ResMed CPAP Error 21: High Leak Detected on AirSense 10

Quick Answer

ResMed's Error 21 is a therapy-specific code tied directly to the AirSense 10's leak detection algorithm, which continuously measures the difference between motor output and the expected mask vent flow rate. The AirSense 10 AutoSet models attempt to compensate for leaks automatically before logging an error, so Error 21 only appears when the leak is both large and sustained beyond the auto-compensation window.

In the field this is almost never a broken machine. It's a silent leak the user didn't notice. If you wake up with a bone-dry mouth or your partner says the machine sounds like a jet engine, that's Error 21 happening. Your therapy's hitting a wall. Most of my service calls for this end with a fresh silicone cushion or a chin strap, because the old cushion lost its spring months ago and nobody noticed.

ResmedCpapSeverity: moderate85% DIY Success

Time to Fix

5–30 min

Difficulty

beginner

Parts Cost

$12 – $55

Tools Needed

CPAP mask cleaning wipes, Mild dish soap and warm water

What Does the 21 Code Mean?

Think of Error 21 as a low tire pressure light for your CPAP. The blower's working overtime trying to make up for air escaping somewhere it shouldn't. Before you assume the motor is shot, you need to look at the consumables first. Nine times out of ten, a thirty dollar part fixes this and saves you from a five hundred dollar machine replacement.

Most Likely Causes

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

Mouth breathing with a nasal or nasal pillow mask40%

Worn or compressed mask cushion losing seal24%

Headgear straps stretched and too loose14%

Mask not fully assembled after cleaning12%

Incorrect mask size (too large or too small for face)10%

Symptoms You May Notice

You wake up with a mouth that feels like you've been chewing sand all night, that bone-dry cotton-mouth is usually the first sign something was wrong.
Audible hissing from the edges of your mask while it's running, sometimes loud enough that your partner can hear it from across the room.
Dry, irritated, or crusty eyes in the morning because air is blowing up from under the mask bridge directly toward your face all night.
Your machine's pressure keeps ramping higher and higher during the night and never settles, that's the AutoSet algorithm chasing a leak it can't catch.
AHI stays stubbornly high in your ResMed app even on nights you wore the mask the whole time.

Can you reset a Resmed cpap to clear the 21 code?

To clear the code on an AirSense 10, hold the Dial and Home buttons together for three seconds to enter the Clinical Menu, then exit. For the AirSense 11, a simple power cycle usually does it. Just remember, resetting is just turning off the alarm. If you don't snug up that mask or replace the worn seal, the code will be back by tomorrow morning.

Tools Required for Diagnosis

CPAP mask cleaning wipesMild dish soap and warm waterSmall basin or bowl for soaking mask partsClean towel or paper towels for drying componentsChin strap (for mouth breathing test)Replacement mask cushion matched to your mask model

Diagnostic Checklist

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

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
Replacement mask cushion for your specific ResMed mask modelN/A · $20–$55	N/A	$20 – $55
Replacement headgear strapsN/A · $20–$45	N/A	$20 – $45
CPAP chin strap (if mouth breathing is a factor)N/A · $12–$25	N/A	$12 – $25

Frequently Asked Questions

What leak rate is too high for a ResMed CPAP?

ResMed machines are smart enough to ignore the intentional leak from your mask vents. Every mask has to bleed some air so you don't rebreathe carbon dioxide. Error 21 triggers when the unintentional leak, the bad kind, stays above 24 liters per minute for several minutes. Think of it as the machine losing more air than it can safely pump to keep your pressure stable. You can see your Leak rate in the Sleep Report menu if you've got the Essentials setting turned on. Anything under 24 L/min on the unintentional side and you're in normal range.

Does Error 21 mean I need a new mask?

Usually just a new cushion, not the whole frame. Cushions should be replaced every one to three months depending on how oily your skin is. If your headgear straps are maxed out and you still can't get a seal, the elastic is shot, and that's roughly a six-month replacement cycle. Replacing these soft parts is standard maintenance and clears Error 21 the vast majority of the time. A full replacement mask frame runs thirty to fifty dollars, a cushion alone is ten to twenty. Always start with the cushion before you buy a whole new mask.

Can I use tape to fix a mask leak causing Error 21?

Don't. Not duct tape, not medical tape, not anything. Those adhesives off-gas chemicals you really don't want to be breathing all night while you're asleep. And honestly, they don't even work well because you sweat, you move around, and the tape fails by 2 AM anyway. If the mask is leaking, it's either worn out, the wrong size, or the straps are stretched. Tape is a band-aid on a problem that needs a real fix. Get replacement cushions from your DME supplier or directly from ResMed.

Can Error 21 mess up my AHI data or insurance compliance tracking?

Yeah, it absolutely can. When your leak rate blows past that 24 L/min threshold, the pressure your machine delivers becomes unreliable. Your AHI numbers for that night are basically garbage because the device couldn't maintain prescribed pressure consistently enough to measure accurately. If you're sharing this data with your doctor or it's being pulled for insurance compliance, a night with Error 21 can look like you weren't actually using the machine at all. Fix the leak first, then give it a solid week of clean nights before any follow-up appointments.

How do I know if the leak is from the mask or somewhere inside the machine itself?

Honestly, it's almost always the mask. CPAP machines are sealed units and don't develop internal leaks. But here's a quick sanity check if you want to be sure: disconnect the hose from the mask end and cover the open hose end with your palm while the machine is running. If you feel pressurized air pushing out around your hand, there's a hose issue. If it feels fully blocked, the machine is working fine and your leak is somewhere in the mask system. Every Error 21 I've ever been called out for turned out to be the cushion, the headgear, or mouth breathing. Don't overthink it.

Related Resmed Cpap Error Codes

Code: HUBResMed CPAP Error Codes: Complete AirSense 10 and 11 Fault Guide Code: 006ResMed CPAP Error 006: Device Alert and Check Mask on AirSense 10 Code: HIGH-LEAKResMed CPAP High Leak Detected: Fix Large Leak on AirSense 10

Models Known to Experience 21 Errors

This repair applies to most Resmed cpaps with this error code. Common model numbers include:

AirSense 10 AutoSet, AirSense 10 AutoSet for Her, AirSense 10 Elite, AirSense 10 CPAP, AirSense 10 AutoSet Card-to-Cloud, AirSense 10 with HumidAir 3i, AirSense 11 AutoSet, AirSense 11 AutoSet for Her

Written by

Sarah Kim

Smart Home & Specialty Appliance Tech · 12 years experience

Last verified for technical accuracy on March 15, 2026