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.
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. The most common cause in practice is a nasal mask user who opens their mouth during sleep - mouth breathing with a nasal or nasal pillow mask creates a leak path that the device cannot compensate for and will not correct regardless of pressure. A full-face mask or chin strap resolves this immediately. Worn cushion seals and incorrect mask sizing are the next most frequent causes. Hardware sensor faults causing Error 21 are rare and only considered after all mask-side causes have been ruled out.
What Does the 21 Code Mean?
Error 21 means the device detected a large, sustained mask leak above 24 L/min. Fix the mask seal before suspecting hardware.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Symptoms You May Notice
- Error 21 logged at end of therapy session
- Waking up with dry eyes (mask air blowing toward eyes)
- Audible hissing sound from mask edges during therapy
- Therapy appears to run but AHI not controlled
- Device increasing pressure continuously without stabilizing
Can you reset a Resmed cpap to clear the 21 code?
AirSense 10: hold the home button for 5 seconds, navigate to Settings > Device > Reset Device. AirSense 11: Settings > My Options > Device Options > Reset. Resetting alone will not fix Error 21 if the underlying mask seal problem is not addressed.
Tools Required for Diagnosis
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
Put on the mask while sitting upright
Put on the mask while sitting upright, start a therapy session, and perform a mask fit check. Press the Start/Stop button to begin airflow without the hose connected to a pillow. Observe the leak indicator on the screen. Adjust headgear until the seal indicator shows green or acceptable range.
Remove and inspect the mask cushion
Check both ends of the CPAP hose
If you use a nasal mask or nasal pillow mask: consider whether
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 · $0–$0 | N/A | $0 – $0 |
| Replacement headgear strapsN/A · $0–$0 | N/A | $0 – $0 |
| CPAP chin strap (if mouth breathing is a factor)N/A · $0–$0 | N/A | $0 – $0 |
Repair vs. Replace Calculator
Frequently Asked Questions
What leak rate is too high for a ResMed CPAP?
Does Error 21 mean I need a new mask?
Can I use tape to fix a mask leak causing Error 21?
Related Resmed Cpap Error Codes
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 CPAP, AirSense 10 Elite, AirSense 10 For Her, AirSense 11 AutoSet
Last verified for technical accuracy on March 15, 2026