Continuing the discussion from Web based rules engine - ready for testing:
I am amazed at how many people try to test a CO detector with car exhaust or gas appliances! Don’t do that. 
Instead, to do a low level test on a unit with a digital readout, just use a lit cigarette held close to the detector and you should see the numbers change. It probably won’t go high enough to trigger the siren.
To do a high level test, use a spray sold for this purpose, but you have to put a plastic bag over the detector for this. Pain in the neck, but it’s the only way to Do a sensor test that will trigger the siren.
Some people are more sensitive (or have pet birds) so may use the sensor at below siren levels (70 ppm).
Be aware that every test reduces the life of the sensor. So don’t go crazy on this.
You should also be able to just push a button on the detector to test that the siren is operational independent of the sensor. That test does not reduce sensor life.
For my own home, I buy a good brand from a retailer with regular inventory turnover, usually Home Depot. I do the button test, but not the CO test. And I replace the detector every 4 years if rated for 5, or 2 years if rated for 3. But everyone’s different in terms of peace of mind requirements.