Synology NAS Error Code 40: How to Fix It

Medium 30-90 minutes Critical Severity Verified June 2026
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Error Code
40
Brand
Synology
Product Type
nas
Severity
Critical
DIY Difficulty
Medium
Estimated Fix Time
30-90 minutes
Synology NAS Error Code 40 indicates that the CPU temperature inside your Synology device has reached a critical level, putting it at serious risk of thermal shutdown, data corruption, or permanent hardware damage. This is a high-priority warning that requires immediate attention — your NAS may throttle performance or shut itself down to prevent damage. Acting quickly can save both your device and the data stored on it.
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Tools You'll Need

How to Fix Error Code 40

  1. Check DSM Temperature Readings Immediately

    Do not power off the NAS abruptly unless it is too hot to safely touch or you smell burning. Sudden power loss can cause data corruption or damage drives. Use DSM's safe shutdown option if a controlled shutdown is needed.

  2. Reduce CPU Load Right Now

  3. Inspect and Clean the NAS Ventilation

    Always unplug the NAS before performing any physical cleaning or inspection. Never use a vacuum cleaner directly on electronics as static discharge can damage components. Use only compressed air designed for electronics.

  4. Improve the Physical Location and Airflow

  5. Inspect the Internal Fan(s)

    If you need to open the NAS chassis to inspect the fan physically, ensure the device is powered off and unplugged. Some Synology models allow fan replacement without tools; others require a screwdriver. Handle internal components carefully to avoid static damage.

  6. Replace the Internal Fan If It Has Failed

    Only use a replacement fan rated for the exact voltage (typically 12V) and connector type specified for your model. Using an incompatible fan can damage the NAS motherboard or result in no fan speed control. If you are not comfortable opening the unit, contact Synology support or a qualified technician.

  7. Check and Reseat Thermal Paste on the CPU (Advanced)

    This step voids your warranty if your device is still under warranty. Only attempt this if you are comfortable with electronics. Applying too much or too little thermal paste can worsen the problem. Use only thermal paste rated for electronics — do not substitute with other materials.

  8. Update DSM and All Packages

    Always back up your NAS configuration (Control Panel > Update & Restore > Configuration Backup) before performing firmware updates.

  9. Configure Temperature Alert Notifications

Parts You May Need

Replacement NAS internal cooling fan
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CPU thermal paste
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Compressed air canister
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Replacement NAS internal cooling fan
Check Price on Amazon
CPU thermal paste
Check Price on Amazon
Compressed air canister
Check Price on Amazon
Replacement NAS internal cooling fan
Check Price on Amazon
CPU thermal paste
Check Price on Amazon
Compressed air canister
Check Price on Amazon
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When to Call a Professional

Contact Synology support or a qualified NAS technician if: your NAS is still under warranty and you do not want to void it; the fan replacement does not resolve the overheating; temperatures remain critically high after all steps above; you suspect a faulty CPU, motherboard, or heatsink that requires component-level diagnosis; or if you smell burning, see scorch marks, or the unit has already thermally shut down multiple times. Data recovery professionals may also be needed if the NAS has already suffered heat-related data corruption.

Frequently Asked Questions

What temperature is too high for a Synology NAS CPU?
Generally, a Synology NAS CPU operating above 80°C (176°F) is considered dangerously high and will trigger Error Code 40. Normal operating temperatures are typically between 40°C and 65°C (104°F–149°F) under load, depending on the model and ambient room temperature. You can monitor this in real time via DSM's Resource Monitor or Info Center.
Will Synology Error Code 40 cause data loss?
Error Code 40 itself does not directly erase data, but the conditions causing it can. Extreme CPU heat can lead to unexpected shutdowns, which may corrupt files or file systems currently being written. Prolonged overheating can also permanently damage the NAS hardware. Act on this error immediately and ensure your data is backed up using Synology's Hyper Backup or another backup solution.
Can I prevent Synology Error Code 40 from happening again?
Yes. Keep your NAS in a well-ventilated area with at least 6 inches of clearance on all sides, clean dust from the vents every 3–6 months, set DSM fan speed to Cool Mode rather than Quiet Mode, avoid running heavy tasks like transcoding or large backups simultaneously, keep DSM and packages updated, and configure temperature alert notifications so you get early warnings before temperatures become critical.
How do I know if my Synology NAS fan has failed?
DSM will typically show a fan failure alert in the Notification Center and on the main dashboard. You can also go to Control Panel > Hardware & Power > Fan Speed Control to check reported fan status. Physically, a failed fan will either make grinding or rattling noises, or produce no airflow/noise at all when the NAS is powered on. If you suspect a fan failure, power off the NAS immediately to prevent further heat damage.
Does Synology Error Code 40 mean I need to replace my NAS?
Not necessarily. In most cases, Error Code 40 is caused by fixable issues: dust buildup, poor ventilation, a failing fan, or dried-out thermal paste. Replacing the fan or cleaning the unit resolves the majority of cases. However, if the CPU or motherboard has already been damaged by sustained overheating, or if temperatures remain critical after all troubleshooting steps, replacement may be the most cost-effective solution for older units.

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