Hey!
Looking for advice on senseair s8 sensor.
After not having been used for quite a while sensor was showing really low values. I checked that ABC is turned on and waited for a couple of weeks – nothing changed – when i opened a window it would still show around 0.
I decided to do manual calibration outside. After this it successfully showed 400 as lowest value for a week or two, but now started to be really low again (see screenshot)
Would appreciate the advice on what to look for?
What is your setting for the ABC period? The default is 7 days. In case it is switched off, the sensors drift will accumulate.
Hey!
Thanks for the fast reply.
It’s switched on and it has the default value set, that’s why it is weird to me
I’ve also realized the screenshot is missing the axes
Double checked and it should be 180 as it showed me in the response.
>>> import serial
>>>
>>> ser = serial.Serial("/dev/serial0", baudrate=9600, timeout=0.5)
>>> ser.write(b"\xfe\x03\x00\x1f\x00\x01\xa1\xc3")
8
>>> r = ser.read(15)
>>> r
b"\xfe\x03\x02\x00\xb4\xac'"
@Achim_AirGradient Hey, sorry for bothering.
I was wondering if there is something wrong with my sensor? I was not home for 4 days and it kind of self corrected from zero to 400 around every few hours, whereas if the ABC period is 180 hours it should not change that fast? Also its was not even 400 ppm
Any chance your HVAC fan was running during those times when it jumped up, either a scheduled run of just the fan, or heating/cooling cycles? That can cause a change in CO2 levels as it circulates air from around the house that could cause it to jump up, and then lower over time until the fan cycles again.
thanks for the answer. I live in Germany, the houses here a well insulated, and HVAC is not a thing really 
Also strange that it showed 0ppm when I opened the window before leaving but then kind of corrected itself to 400
If it hadn’t ran the automatic calibration in awhile, or if it was in a well sealed house for over a week, then I’ve seen it baseline calibrate to some false high number. Then when you do actually move it to a low CO2 environment, it could go down as long as 0.
Imagine a home where the actual indoor CO2 never goes below 800 due to low air exchange and multiple people. After a week the Automatic Baseline Correction runs and sets that to be considered 400 on the sensor, although it is really 800. Now if you move it outdoors where it is actually 400, the sensor would start to report 0 or a very low number until the next week when does ABC again based on the lowest value it observed
Just to make sure I understand correctly how the calibration works: assuming you have ABC turned on, when the time comes, the lowest value from the last 8 days is used to calibrate the sensor?
Correct, the lowest value since the last ABC is now treated as 400.
As my home almost never gets all the way down to 400, I’ve chosen to disable ABC and every few months I take it outside for 10 minutes, ABC, 5 minutes and then back inside. I just did it in the last few weeks and after being indoors with no ABC for 3 months, it was off by around 100
I think in my case ABC is somehow just not working :(. On the image that started the thread i’m showing that the sensor was reading low, then i did the manual calibration, but after a week it started reading low again. Don’t think that something changed externally as air in Berlin is the same and i am venting daily to prevent mold.
I think i’ll follow your path and just disable ABC altogether.
You are using the stock firmware, right? For giggles and grins, try going here and see what your Uptime Count is showing
AirGradient
It should be in the thousands, but if it is pretty low, that could mean it is rebooting frequently. It shouldn’t change your CO2 levels like you are seeing, but PM2.5 or VOCs can show odd behavior if it is rebooting.
Your graph looks like HomeAssistant, so if you are using Stock firmware, go into Settings>Devices and find your AirGradient and see what “Configuration Source” is set to. It defaults to Local when you first add it, meaning it doesn’t use the Dashboard settings, so maybe you expect ABC to be disabled but it isn’t.
You could try changing it to Cloud so it for sure uses the Dashboard settings.
And if you are using ESPHome firmware, then everything is different.
to be truly honest, i’m not using Air Gradient at all. I was inspired by it, but only wanted a small subset of the features, so bought just a co2 sensor form a local vendor. It’s connected to my Raspberry via serial, I checked that raspberry is not undervolted though!
Thanks 
@Achim_AirGradient, would it be possible to expose via the local API when the last calibration happened? Then it’d be easy for someone to check the min CO2 value from the previous 8 days (going back from that timestamp) and so one would have a clearer view against which value the calibration occured. Does that make sense?
Just wanted to shared un update is someone has a similar problem.
In my case disabling ABC and calibrating the sensor manually by placing it outside for a bit and triggering calibration pin solved the issue. Even after 4 months I see around 400 when the windows have been opened for quite some time.
Assuming one has the AG device in Home Assistant, here’s one way to check the minimum value*:
sqlite3 -readonly /path/to/your/home-assistant_v2.db "select min(cast(state as integer)),datetime(last_reported_ts, 'unixepoch', 'localtime') from states where attributes_id is (select attributes_id from state_attributes where json_extract(shared_attrs, '$.friendly_name') == 'I-9PSL Carbon dioxide') and state not like 'unavailable';"
This looks at the data of an indoor model (I-9PSL). If one wants to look at an outdoor model, I-9PSL should be replaced with O-1PST. I only have one indoor and one outdoor models, so this query works for me. If one has more indoor/outdoor models, the inner SELECT would need to be adjusted accordingly.
When I run the above, I get 443|2025-10-19 21:07:12 for my indoor sensor and 397|2025-10-19 18:22:43 for the outdoor one.
*the HA db stores data for the last 10 days, so there might be some corner cases where the reported minimum value was NOT in the last 8 days. Or you can get fancier with the SELECT and restrict the query to the last 8 days.