TL;DR: If your whole family is cranky on a road trip, crack a window, or switch to manual HVAC mode with recirculation off. Auto mode keeps recirculation on a lot of the time, resulting in stuffy, high-co2 air.

There was more than a little skepticism on Reddit a couple of days ago when someone re-posted this Facebook post where a Mach-E driver noticed that CO2 concentrations get quite high in the cabin with “Auto” HVAC:

The original post claimed that they measured CO2 concentrations past 2000ppm with HVAC set to auto and recirculation off, suggesting that “Auto” actually does enable recirculation a significant portion of the time, silently. The reddit response was very reddit (totally hyperbolic in mostly the negative direction) but the original post seems totally reasonable to me – high CO2 concentrations, while not hazardous, do make you sleepy, slow, and fatigued, and are definitely not ideal for long durations operating a motor vehicle! To me, it seemed totally reasonable but also very curious, so I tested it.

From a few years back I have an Adafruit Edge Badge laying around with custom firmware to read and plot a Sensirion SCD30 NDIR CO2 sensor (±30ppm ±3% accuracy). The white readings on the left are plot scale, the green is the current reading, and red is the highest achieved on this power cycle.

First Test: Validation

I have a 45-60min commute so I decided to test during that. My first goal was not to reproduce the findings but to validate whether CO2 could plausibly get that concentrated in the worst case. I drove to work with my windows closed, HVAC on, and recirculation enabled, as a worst-case scenario.

Even just 10min into the drive, CO2 had already climbed to 1133ppm. I calibrated that morning in fresh air to 430ppm.

Upon opening the door to drop off a package, CO2 immediately dropped to within margin of ambient background:

15min into the next segment, it was back up to 1500:

I opened the front window all the way and even on the highway concentration immediately dropped down into the 500 range. It takes the sensor a minute to respond, so that curve is over the course of about 2min (I believe 5min is shown on screen):

Closing the window, concentration immediately increased again:

Test 2: Manual Mode

The second test was to see whether manual mode resulted in external air where as the author claims auto didn’t. I turned on the HVAC, manually selected the face and floor vents to disable auto, and made sure recirculation was ON. I let the CO2 concentration climb to a reasonably high value over maybe 10min, then turned recirculation OFF, and it immediately dropped. So we can definitely say that the cabin is pretty well sealed with recirculation on, and has decent air exchange with it off!

Test 3: Full Auto

Test 3 was to see whether, as originally posted, auto mode would result in super high CO2 concentration even with recirculation turned off. I set the HVAC thusly: auto with no recirc.

18min in, already 1450ppm

After 33min on the highway, I hit a peak at 2178ppm, and concentration started dropping. Not very fast – the scale is a little deceptive since it’s automatic. This is only a 100ppm drop. Notably, I used my washer fluid around this time and while I HAVE noticed a smell of alcohol with this fluid, I DID NOT notice it this time. So it stands to reason that recirculation was enabled even though I didn’t do that myself, and it shows as off.

A few minutes later (the first little dip in this image is the big dip in the image above), I noticed two things: I smelled heavy diesel exhaust from some other vehicle on the highway near me, and CO2 started dropping very fast. So I suspect at this moment about 38min into the drive, Auto HVAC first chose to open the fresh air path.

Concentration dropped to and leveled off at about 1000ppm as I got off the highway close to work…

…and then started tipping back up again by the time I parked 5min later.

Conclusion

I believe I have completely validated the original Facebook post. This is not dangerous by any means – there’s hardly some suffocating flaw with the car. I’m very sure Auto is prioritizing energy management over cabin comfort by closing the external air mixer. But that’s very interesting to me, and will almost certainly change how I drive the car, preferring manual over auto and trading fresh air quality for a little energy savings. For what it’s worth, mid-2000s PPM seems to be a ceiling. Either by active air quality monitoring or by a simple timer, I don’t know, Auto WILL eventually mix in fresh air to keep it from getting TOO stale.

I can definitely imagine this mattering a lot if you’ve got a full car on a long trip. CO2 in concentrations this high isn’t dangerous, but will definitely make you drowsy, slow, and irritable. I conclude that cracking a window could really help keep the peace in this car.