I arrived home today from running some errands to find that the carbon monoxide alarm in my garage was going off. The display was reading 55 ppm CO (which I gather is a non-lethal but nevertheless high reading).

The garage is detached from the house and has no heat. There are several pieces of gas-powered lawn equipment stored there but nothing has been running. I was gone from the house for several hours so I assume any exhaust from the car briefly running in the garage had dissipated. (When I left the house I just started the car and drove out... no remote start or idling in the garage.)

Does anyone have any idea what the cause of carbon monoxide might be? The CO alarm is less than a year old. Is it defective?


3 Answers 3


The answer is fairly obvious to me. For one thing, 55 parts per million isn't "high".

When cold engines first start, they run rich," Greiner said. The catalytic converter is cold and not converting deadly carbon monoxide (CO) to carbon dioxide (CO2). Concentrations in the exhaust can be more than 80,000 parts per million. Concentrations so large fill the garage with carbon monoxide in a very short time even with the door open. ...after only two minutes of warm-up in the opened garage, carbon monoxide concentrations rose to a lethal 575 parts per million


  • 1
    I guess maybe this really is the answer, but I was surprised that the concentration was registering at all several hours after leaving. The car was probably only running for ~10 seconds before I pulled out of the garage and I was gone for at least 3 hours. And I realize that 55 ppm isn't really that dangerous, but it was enough to trigger the CO alarm.
    – Hank
    Dec 12, 2015 at 3:26
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    In a household situation, 55ppm could indicate a very serious accumulation problem with a furnace or other appliance. Virtually zero CO should be present in a home. In a garage, it's peanuts.
    – isherwood
    Dec 12, 2015 at 3:39
  • I guess I just thought it was weird since I've had the CO detector installed for almost a year and I've never heard a peep out of it or seen anything but "0 ppm" on the display. But maybe it's just the cold weather changing the chemistry of the engine.
    – Hank
    Dec 12, 2015 at 3:50
  • @HenryJackson - I agree it seems strange you'd have zero all that while and now that there's a reading on there all of a sudden. I might swap the CO in the garage with one of your other ones and watch the readings and the max on them both. There are a lot of junky CO detectors out there. Dec 12, 2015 at 11:09

isherwood's answer is good. Check out this information about why you shouldn't put a CO detector in your garage. The link below has lots of other useful information, as well.

Can I place a CO detector or alarm in the garage to warm me of excessive CO? No. Residential detectors are not designed or approved to operate in garage conditions. Temperatures and humidity variations are too large, and the high start-up amounts of carbon monoxide can easily damage or destroy the sensing cell. CO detectors should never be placed or used in the garage."


  • It seems silly to say that CO sensors should "never be placed in a garage". Sure the stakes are a lot lower and if the alarm goes off you can just open the door and walk away, but it's not like CO poisoning doesn't count if it happens in a garage. I would sure as hell rather have a false alarm in the garage than no alarm at all.
    – Hank
    Dec 12, 2015 at 3:29
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    The point is that CO is almost always present in a garage, and if you're not smart enough to keep running vehicles well ventilated, a little beeping device probably won't help much.
    – isherwood
    Dec 12, 2015 at 3:38

I actually just had a very similar situation of the CO monitor in the garage. No recent car usage, and the CO sensor suddenly went off at ~50ppm.

EMT and gas company came to check, and ultimately the source was a power transformer and a charging battery. All other likely sources (furnace, water heater, drier, oven, gas lines, etc.) were checked and conclusively shown to not be the source(s).

For reasons that I don't completely comprehend, the electronic devices either offgas CO or Hydrogen gas (which is then misread by the CO detector). Both of the charging battery and power transformer were a very slow sources, but built up in the garage over time until the alarm set off at ~50ppm. Unplugging the power transformer and the battery immediately resolved the problem. I was told by the gas company that they see this infrequently but not rarely: charging batteries are an unlikely but valid reason to have CO detectors go off.

I would not recommend determining with a commercial/home use CO detector alone if that is your root cause. I would recommend relying on a professional or an industrial CO detector to determine if that is your root cause.

  • I take it the culprit battery was a lead-acid type? Dec 27, 2022 at 12:36

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