I received this product today. It is wonderfully small, just 1.75" x 2.5" x 5/8". I am intrigued by its size and high-quality construction, and after reading the positive reviews here on Amazon, I think this little unit will probably work great for detecting CO in my car. It is very easy to use, with only one button and a nice feature set for detecting CO in a vehicle, and works right away out of the box. No muss or fuss.NOTE: This product is not designed for use in a home. Home units have higher CO thresholds (to avoid too many alarms) and much louder alarms so they can be heard at a distance. This detector is specifically designed for use in a small space like a vehicle.The detector I received came in a very small box measuring just 2.25" x 3.25" x 1", and included a very tiny one-sided sheet of instructions measuring 4" x 5.5" (attached to this review). I saw a YouTube review of this product (https://youtu.be/ffGV4ORnq8U) from Dec. 2017 that shows the exact same product in a much larger box than I received, with multiple, full-size pages of instructions, but I like the small box (save trees!) and the tiny instruction sheet I received seems to cover the required information. Weirdly, the manufacturer does not seem to have a web site (!), so all the available product information is simply on the small instructions sheet, the Q&A and reviews here on Amazon (and elsewhere?), and the one YouTube review I found (above). So, in case it is useful to someone, below I have collected all the info I've found on this product:1. This detector comes with a convenient plastic clip with a peel-off 3M stick-on mount for attaching to a surface in your vehicle (if you want to attach it). It is slightly tricky to remove the detector from the plastic clip mount the first time you try it, but it gets easier with practice. Also you don't need to remove the detector from the clip mount very often because the battery lasts a long time (see below), unless you are tracking down an active CO leak and you need to keep resetting the max CO level reading (see below).2. The instructions say don't put the detector in direct sunlight, because it will not work correctly at temperatures above 122F. Above that temperature the unit will display "---" until the temperature drops below 122F.3. You can use the detector right out of the box. The battery is installed and ready to roll.4. To turn on the unit, hold the power button for 3 seconds. The display will read "CAL" (meaning "calibration") for 3 minutes, it will make a short beep to indicate that warm-up calibration is complete, and then it will show the CO reading (which will normally be 0). To turn it off, hold the power button again for 3 seconds until the display goes blank. If you turn it back on again, you will see the "CAL" message again for 3 minutes.5. The detector is designed so that you can leave it "on" all the time, unless you want to clear the maximum CO reading (see below). The instructions say it will run 8-12 months on the included battery, and then you can change batteries. By the way, it uses a common CR2032 button battery, very easy to find in the drugstore or whatever (the specific type of battery is not mentioned in the instructions). When the battery is getting low, the instructions say the unit will "beep beep" every 30 seconds.6. The detector displays a continuous CO reading that you can glance at any time.7. If the detector has a continuous CO reading of at least 9ppm for at least 60 seconds, the red LED light will start blinking, and you can always read the actual CO level on the detector display.8. If the detector has a continuous CO reading of at least 25ppm for at least 60 seconds, the buzzer will sound in addition to the blinking red LED. (I don't know if the buzzer is an intermittent beep or a continuous buzzer sound, but multiple reviews here have reported that the buzzer definitely gets your attention.)9. The tiny instruction sheet (attached) says in the introduction that this detector has a third alarm mode that goes off when levels exceed 50ppm for an extended period, but there is no mention of this third alarm mode in the rest of the instructions or in the feature list here on Amazon. I don't know if there actually is a third alarm mode, or if that info was put in the instructions by mistake, but in any case, the 25ppm alarm will of course cover the 50ppm case as well.10. To view the maximum CO reading since the last time the unit was reset, press and release the power button briefly, at which point the blue "MAX" LED will come on and the maximum CO level detected will appear on the display for 5 seconds, before it resumes normal operation. You can repeat this as often as you want. The act of reading the maximum CO level does NOT reset the max level reading back to zero; it is still recorded inside the unit.11. If you want to clear the maximum CO reading, to set it back to zero, you have to turn off the detector, turn it back on again, and wait 3 minutes for the "CAL" message to finish and the unit to beep (see above). This reset process is a little inconvenient if you are trying to do multiple tests to see if a CO leak has been fixed. But that situation hopefully won't come up for you very often.12. All CO detectors have an "end-of-life" date, after which the detector needs to be serviced at the factory or discarded and replaced, because it will no longer give reliable CO readings. This limitation is inherent in the technology of CO sensors. This detector is no exception. The unit I received in June 2018 has a sticker on the back that say the end-of-life is Jan 2021, which is 2.5 years from now. There is no mention in the instructions, or anywhere on the Internet that I've found, that says what happens when this particular detector reaches its end-of-life. I assume that it keeps behaving normally, except that you can't completely trust it's CO readings, which is slightly worrisome. Another reviewer here on Amazon mentioned this same concern. Also, some CO detectors have a way to mail the detector back to the company when it reaches end-of-life to have a new CO sensor installed and tested at a lower cost than simply replacing the unit, but there is no mention of that service for this detector. I'm just going to have to somehow remember to discard this CO detector after Jan 2021. Here is my feature request for the next version of this detector: At end-of-life, display "EOF" on the display and do a "beep beep" every 30 seconds, so that the purchaser knows that the detector is no longer reliable and should be replaced.That's it. I'm expecting this CO detector will be very useful in my car.