Which? apparently is a paid service. That I find morally questionable - telling you that some are literally deadly and then asking for money to tell you which ones are.
[1] https://www.which.co.uk/reviews/carbon-monoxide-detectors
[2] https://www.which.co.uk/reviews/carbon-monoxide-detectors/un...
[1] https://www.which.co.uk/reviews/carbon-monoxide-detectors/ar...
"Compared with Best Buy carbon monoxide detectors, the Don't Buy alarms that we've named and shamed on this page just can’t be relied upon to detect the gas and sound the alarm. In our tests, the Don’t Buys failed to detect carbon monoxide in more than one-in-three of our tests. This adds up to 26 failures out of 72 tests. By contrast, the Best Buys passed all 312 of our carbon monoxide gas tests.
The potentially deadly CO alarms all claim to have passed the relevant EU safety standard, EN 50291. But repeated failures in our lab tests – which mirror those of the safety standard – lead us to doubt that they actually did. And we’re as concerned about several lookalike models as we are about the three models that failed our tests.
To find out which models are Don't Buys, as well as our Best Buy carbon monoxide detectors, sign up for a £1 trial to Which?"
https://familylivingtoday.com/best-carbon-monoxide-detectors...
or
https://www.consumerreports.org/products/carbon-monoxide-det...
(but you've got to pay for that one)
Or, simple rule of thumb, stick with one of the major brands like Kidde or First Alert (other reputable brands like Nest should be safe too). Or better, use more than one major brand/model just in case there's a design flaw in one of them.
If you don't want to pay for Consumer Reports: their top rated standalone detector is the First Alert CO615 ($30)
I bought mine from Home Depot, who I hope has enough control over the supply chain from the manufacturer that they are legit.
But counterfeit products is another reason to go with multiple brands/models, you're less likely to receive perfect counterfeit replicas of each type.
As you can see in this CO detector case, when there’s something more serious than whether it gets your whites whiter than white, they share it with the press and make a campaign around it.