Question:
I am also looking for a smoke / co tester for use in the garage - any comments on good brands or problems with these?

Answer: Yes. Don't use them in a garage. Smoke detectors of any sort should not be installed there. They need to be located in a controlled environment (temp / humidity) that is low on dust and does not routinely have smoke in it. A garage does not meet these requirements.

Also, while some people like them, I don't believe you should combine smoke and CO detection on a single output. The response to smoke detection is to alert the family, then check the place for signs of fire. If no fire is detected you can reset the alarm and go about your business. With CO it is not the same. The proper response (IMO) is to alert the family, exit the premises immediately and have the fire department check before re-entering.

Humans cannot see or smell CO gas. The only indication that there is lethal gas accumulating in the house will be that CO detector. If you decide everything is OK and go back to sleep when in fact there is a CO problem you may never wake up.

During the years I operated a small alarm company I had several real CO events where the homeowners did not believe there was a problem. In at least two of those the reason they survived is we called the fire department in spite of their protestations that all was well.

In an earlier thread I discussed one of these events. Another time a family I protected was having a New Year's party. They had numerous guests in their large, brand new (well sealed) home. The gas furnace was running because it was extremely cold outside. The home had two large fireplaces and each had a roaring fire going.

We had installed four CO detectors -- one in the furnace room, one on the main level and two on the bedroom level. All of them triggered, sounding the alarm and triggering the signal to the central station. We called the FD and then rang the house (our policy for CO alarms).

The homeowner insisted that all was well but that the detectors were just "going crazy" because everything seemed OK. When the FD arrived they brought in a tester and informed the residents that the house was full of CO gas. They put out the fireplace fires, opened several windows and doors and after a while the air was clear.

The problem, BTW, was the fireplaces. The flame in a furnace is a very small fire compared to a fireplace. The two fireplaces were drawing so much convective current up the chimney flues that it was being replaced *down* the furnace flue. This allowed CO to pour into the basement. The forced air heating system drew some of its air in through a basement return and distributed the CO throughout the house.

If the CO alarm had been combined with fire the homeowners might have thought it was a smoke alarm and, finding no fire, dismissed the matter with catastrophic results. It is because of several experiences like this that I don't recommend using combined smoke/CO detectors.