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What level of carbon monoxide (CO) is considered normal inside a car?

My 2002 VW Passat was releasing fumes into the car and making me dizzy. I went to a VW dealership for the repair. It turned out to be oil and coolant leaking onto the exhaust. No exhaust leaks were found (checked by 2 mechanics).

The smell is gone, but I still feel dizzy from time to time. Given the health risk, I purchased a carbon monoxide detector (Fieldpiece SCM4 Carbon Monoxide Detector) to ensure exhaust fumes weren’t entering the car.

On a cold start, the detector reads a max. of 70 to 230 parts per million (ppm) for a short period of time (1 minute), then returns to 0ppm (screenshot: 2019 01 28 - Carbon Monoxide Reading.png).

Should I be concerned?

Given that car bodies are not completely sealed from the outdoors, I believe a small amount of CO for a very short period of time is normal, especially at startup in cold weather when gas isn’t burning efficiently. Is that correct? My CO detector just registered 18ppm for about 30 seconds in a 1-year-old BMW X1 in -10°C weather.

Thank you in advance to anyone willing / able to help!

I dunno. My home one is supposed to alarm at 100 but never reads over 0. Yours sounds pretty normal. If you start the car in the garage there can actually be a back draw a little with the door open.

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Good to know. Thanks Bing!

YES you should be concerned.

The device you have is sorta OK, it is not up to industrial standards, but better than a home CO detector. It’s sensor is used in industrial devices, but the calibration procedure is for the birds. It displays a number, but it is not traceable to a known standard. But, if you are picking up readings of up to 230 ppm you have a really bad problem. Assuming the detector has accuracy of +/- 50% you have anywhere from 115 to 460 opm CO. OSHA says you cannot be exposed to an average of 50 ppm for 8 hours, (NIOSH is lower 35 ppm) OSHA says 200 ppm exposure for no more than 15 minutes in a work day. This device is telling you that you have CO and sometimes lots of it. Oil or coolant on the engine will not give much CO that ends up in the passenger area, but it sure will stink it up. Dizziness is a symptom of CO poisoning. You and your dealer have more work to do to figure this one out.


I will call my VW dealer in the morning. Thanks @SteveCBT! I really appreciate it.

@SteveCBT I got the impression it was reading 0 after one minute.

Many areas of the US have independent VW specialist shops that are just as competent as the dealership’s mechanics, but that charge far lower prices. You are certainly entitled to make your own decisions, but I am having a hard time figuring out the logic of taking a 17 year old vehicle to a dealership for diagnosis and repair.

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@Bing Yes, that’s correct - the high levels of CO last for just a minute or so… but a minute every day will add up over time.

Call your local fire department and see if they can suggest a place to get the CO in your car tested at start-up. Maybe they would do it.

I meant with a standardized sensor.

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Try that same experiment with another car, maybe even one at the VW dealership. It sounds like more CO than I’d expect during a cold start, but maybe they all do that. Since it goes away to zero in just a minute seem unlikely that would be the cause of your dizziness in any event. Could be something else in the air though. You could buy a respirator like is used for painting, the kind to reduce breathing the volitile solvents from the paint, those will reduce the amount of CO I think. Could provide a clue anyway. Physicians have a tough time figuring out patients w/dizziness symptoms b/c there are so many possibilities, and not everyone even agrees what dizziness means. Does it mean light-headed? Or does it mean you feel like you are losing our balance? etc etc

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I did try the experiment with another car, but I will try with a few more. By “dizziness”, I am referring to feeling light-headed, a bit off balance, and unable to think clearly. Thanks for the advice George.

That sounds like me all the time … lol … seriously, that’s the difficulty w/diagnosing a dizziness symptom, the sensation is difficult to describe. Most physicians will tell you if there’s no other clear cause (like car exhaust) it is an unknown virus attacking the inner ear system, and may go away on its own, or you have to just live w/it.

Unfortunately civilian respirators do not scrub CO, the link below shows a type of scrubber for CO. An ER can draw blood to check for CO, time consuming and $$ depending on insurance, but the only reliable accurate method to check for human CO exposure.

I am not sure of the range of an exhaust emission analyzer, but that may be worth asking your dealer or shop to see if they can put the probe in the passenger compartment for a quick test.

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Actually they look at cardiac etiologies a lot too. An abnormal heart rhythm can cause dizziness.


The VW technician cleaned out the fuel injectors on Friday and I haven’t detected CO since, but I still felt dizzy after driving this weekend.

I just went to the Fire Department (thanks @jtsanders) to double-check that my detector isn’t a dud. Their professional-grade CO detector didn’t register any CO either. Temperatures have risen drastically (5F to 50F) since my original post, but it appears that CO isn’t entering the cabin anymore!

However, the fireman did notice a slight smell when I turned on the heat. He described it as “unclean air… like the air in some airplanes.” He recommended that I have the cabin filter changed. His theory is that fumes / pollutants are trapped in the filter from the previous oil and coolant leaks (mentioned in my original post).

So whenever I turn on the heat, air is flowing through the dirty filter into the cabin and making me dizzy. This sounds logical to me. Any thoughts? I will ask VW about this possibility tomorrow and keep you posted. Thanks again for your help!

FYI I will find a cheaper independent VW mechanic going forward (thanks @VDCdriver), but the dealership mechanic is already very familiar with my car / issue and hopefully they will replace the filter free of charge given my recent business.