This article’s purpose is to help you identify and resolve any issues you might be having with collecting data from your PurpleAir sensor with a microSD card. To get an overview and learn about what expected and normal SD logging behavior looks like with our sensors, refer to our SD Card Overview Article.
Quick Links: If you’re experiencing a specific issue, jump directly to:
Identifying Your Sensor’s SD Status
If you’re having an issue with SD data collection and suspect that your card is not detected, this section will help you identify which issue you may need to troubleshoot.
When looking underneath your sensor, you should see some LED lights. If you don’t see any, your sensor may not have power, and you should follow our article on Power Troubleshooting.
SD Detected LED Indicators
If your card is detected, you should see a blue LED flash every two minutes, as shown below.
If you’re still having SD issues despite your card being detected and the sensor receiving a stable power supply, proceed from the Incorrect Dates or Timestamps section to see if your specific issue is covered in this article.
SD Not Detected LED Indicators
If your card is not detected, you should see a blue LED flashing three times in quick succession, as shown below.
If this occurs when a card is inserted, we’ll first check whether the logging hardware responsible for detecting your card is itself detected, or whether only your card is not detected. To do so, follow the steps below to identify your sensor’s SD status.
Verifying Your Sensor’s SD Status on our Map
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Connect your sensor to WiFi and register it on the map Sensor WiFi and Registration Guide
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Locate and click your sensor on the map
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Hover over the SD card indicator as shown in the images below
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If you see the message in the left image, proceed to SD Card Not Detected
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If you see the message in the middle image, proceed to SD Logger Not Detected
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If you see the message in the right image, your real-time clock may be disconnected. Contact us at contact@purpleair.com for further troubleshooting
What if I can't connect my sensor to WiFi?
If you’re unable to connect your sensor to WiFi, you can still check the SD status of your sensor. You’ll need to install our PurpleAir Utility software as instructed in this article.
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Follow the section in the Utility article linked above to “Connect and View Sensor Logs”
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Once you see the logs, there should be a hardware string that looks something like:
3.0+OPENLOG+31954 MB+RV3028+BME280+BME68X+PMSX003-A+PMSX003-B
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The “31954 MB” part of the string tells you that your SD card is being detected (this is referring to the capacity of the card). If you don’t see something like this in the string, proceed to SD Card Not Detected
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The “OPENLOG” part of the string tells you that your SD logger is being detected. If you don’t see something like this in the string, proceed to SD Logger Not Detected
What if I see different LED behavior?
Below is an expected sequence of LED behavior for our sensors:
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Upon plugging in your sensor, its SD LED should flicker a blue color a few times
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You may also see a green color flicker once; this is normal and indicates that the SD logger is initializing or that the sensor is rebooting
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You may then see a blue flicker every 2 minutes if the card is detected, or three blue consistent flashes if your card is not detected
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If your sensor is not a Classic or Classic Plus, it will have an AQI light with varying possible colors depending on your air quality, which will correspond with the EPA’s AQI chart, as seen on this page. This will be separate from the solid red LED power indicator, the flickering blue LED SD indicator, and the flickering green indicators
If the LED behavior you’re seeing isn’t described here, refer to this article’s section on Troubleshooting Unusual LED Indicators
SD Card Not Detected
If your sensor’s SD card is not detected as indicated by your SD status indicator on the map, follow the steps below.
1. Ensure Proper Insertion
Remove and reinsert the microSD card. You should feel and hear a click when it is fully inserted. Cards can be inserted halfway—make sure it’s fully seated.
If you cannot hear or feel a click when inserting the SD card, or if the card doesn’t stay in place, the SD logger’s latch may be broken.
Another possible issue, though very rare, is that the SD logger is inserted into a defective plastic bracket. This was corrected in May 2023, so only sensors manufactured before that date can potentially be affected. If this is the case, you can try securing the SD card with tape as a temporary fix, or reach out to us at contact@purpleair.com for a replacement SD logger.
2. Power Cycle with Card Inserted
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Unplug the sensor for 30 seconds
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Plug it back in and watch the LED behavior
If that doesn’t work, try removing the SD card, power cycling again, then reinserting the card.
3. Verify Card Meets Requirements
Ensure that your microSD card meets our requirements.
4. Check Your Card’s Current Capacity
Our sensors may have difficulty detecting microSD cards that are nearly full of data. If your card has significant data on it, you can follow these steps:
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Remove the SD card and back up all data to your computer
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Either delete old files from the card or replace it with a larger card (maximum 32GB)
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Reinsert the card and power cycle the sensor
5. Reformat Your Card (this will erase the data on your card)
If none of the previous steps have resolved the issue, you can try reformatting your card as FAT32 using default options, as this may resolve an issue such as corruption, even if your card was already formatted as FAT32. Ensure that you’ve backed up or transferred the data on this card, as reformatting will erase all data on the card.
6. Try a Different Card
Test with a known-working microSD card that meets our requirements referenced in step 3. If this card works instead, the original card may be damaged or defective.
SD Logger Not Detected
If your sensor’s SD logger hardware is not detected as indicated by your SD status indicator on the map, please contact us at contact@purpleair.com.
Incorrect Dates or Timestamps
SD data timestamps may appear to be incorrect for several reasons. Below are some troubleshooting options ordered from most to least common.
Time Zone Differences
Dates are recorded in UTC, not your local time zone. Be sure to account for the difference between UTC and your time zone when reviewing timestamps.
For example, if you’re in US Pacific Time (UTC-8), data from 3:00 PM local time will show as 23:00 (11:00 PM UTC).
This is the most common reason data appears to be off by a few hours.
Real-Time Clock Not Synced
SD-capable sensors include a real-time clock (RTC) to keep track of time even when powered off. If your data shows dates like January 1, 1970, or January 1, 2000, the RTC is no longer synced. To resolve this, perform the following steps:
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Connect your sensor to WiFi Sensor WiFi Connection Guide
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The sensor will automatically synchronize its clock via the internet (using NTP — Network Time Protocol)
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Confirm whether this sync was successful by checking your sensors’ WiFi status indicator for NTP. You can read about how to do so in this article about WiFi Status Indicators
Please note that previously written timestamps on the SD card are not updated when time is synced. Only new data will have correct timestamps.
Clock Drift Over Time
Even if your RTC was previously set correctly, not being connected to WiFi for extended periods can cause the clock to drift out of sync gradually. The internal RTC does not remain perfectly accurate on its own, and without regular access to NTP servers over WiFi, timestamp accuracy will degrade over weeks or months.
To prevent this, we recommend connecting the sensor to WiFi periodically (even temporarily) so it can re-sync the clock and maintain accurate date and time records on the SD data.
Depleted RTC Battery
Another potential cause is that the RTC battery has failed or is fully depleted. This small internal battery powers the clock when the sensor is unplugged or loses power. Indicators of a dead battery are:
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The clock resets whenever the device is powered off
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Timestamps may revert to 1970 or 2000 every time power is lost
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Even reconnecting to WiFi may not maintain the time setting without replacing the battery
If you suspect the RTC battery is depleted:
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Remove your microSD card and move its data elsewhere
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Reinsert your microSD card into the sensor
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Power cycle the sensor by unplugging it for 30 seconds and plugging it back in
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If you still see incorrect dates after all of the previous steps, contact contact@purpleair.com with your sensor’s Device ID, and let us know you’ve tried all the steps in this guide
Missing or Incomplete Data
If you can see some SD data, but there are gaps where data should exist, the missing data may be recoverable.
For help with determining why your data may be missing, as well as possible strategies to recover it if possible, see our guide on Troubleshooting Missing SD Card Data
For strategies to mitigate the impact of data loss in the future, see our article on Data Collection and Reliability
Troubleshooting Unusual LED Indicators
Blue LED Flickers More Frequently Than Every Two Minutes
The blue LED flashing means data is being written to the SD card at that moment. If the blue light flashes more often than every two minutes, the sensor is outputting supplemental status information, such as WiFi connection events or other information that is not necessary to record.
This excessive logging will fill your SD card much faster than expected, potentially causing data loss. If you see this behavior, follow the steps below:
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Ensure that your card still has space for data collection
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Reach out to us at contact@purpleair.com and let us know you’re experiencing this issue
Green LED Frequently Flickering
A green LED flickering once upon plugging your sensor in or inserting a microSD card is normal, and indicates the SD logger is initializing or in a bootup sequence. If the green LED is flickering frequently, this may be indicative of your sensor crashing repeatedly.
In this case, we recommend going here to download our PurpleAir Utility tool, as this can provide information on your sensor’s behavior. This article details how to copy the logs from your sensor with the tool, which you can then send to us at contact@purpleair.com so that we can assist in the troubleshooting process.
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