Home » How Your Car’s ‘Smell Sensor’ Works

How Your Car’s ‘Smell Sensor’ Works

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Car enthusiasts, particularly those that dabble in keeping older, more experienced vehicles alive, all know that feeling. You’re out on the highway, and you smell something oily, or maybe you smell raw fuel. You’re trying to figure out whether or not that’s coming from your little project, or someone else’s. But what if your car stopped you smelling those nasty things entirely? Well, some cars have a special sensor (which I’m just going to call the “smell sensor” for headline purposes) designed to do just that. Today we’re going to learn about how it works, in fine detail. I even cut one open to look inside!

The sensor goes by a lot of different names. Fundamentally, these are sensors that are able to detect the presence of certain gases in the air. However, automakers like to call them different things. SEAT calls it an air quality sensor, while Volvo calls it the Interior Air Quality Sensor because that sounds even better. However, it’s actually a bit of a misnomer, as we’ll find out shortly, because the sensor actually detects gases outside the cabin. BMW, one of the first companies to implement this technology, calls them AUC sensors. This stands for Automatische Umluft Control in German, which translates to automatic recirculation control in English.  Thanks to BMW’s highly descriptive nomenclature, that tips us off as to how these sensors help keep a car smelling fresh inside.

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At their heart, these sensors are charged with detecting the presence of certain gases in the air. Typically, they detect carbon monoxide, nitrogen oxides, and hydrocarbons. The former two are typical products of the combustion process, while unburnt fuels or smoky oil vapors would be the prime example of hydrocarbons in the air out on the road. If you’ve ever driven past a badly-running diesel or a car with a nasty oil leak, you’ve probably suffered the hot, unpleasant smell indicative of a vehicle working its way towards the crusher.

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The E90 model BMW 3 Series places the AUC sensor just by the base of the windshield where it is exposed to the flow of ambient air.

The sensor is usually located somewhere in the front of the vehicle under the hood, and is thusly used to control the HVAC system to avoid these nasty gases making their way into the cabin. These sensors are typically only fitted to vehicles with higher-end automatic HVAC control systems. In duly equipped vehicles, the air quality sensor feature is enabled via setting the recirculation control to automatic or just generally putting the HVAC in full auto mode. Then, when the sensor detects something unpleasant outside the car, the HVAC system will automatically shut flaps to enter recirculation mode so the nasty air doesn’t enter the vehicle.

So far, so simple. But this is The Autopian, where we like to dive a little deeper. Thus, I grabbed one of these sensors and cut it open to try to find out what lurks inside. I selected an AUC sensor used on my 2008 BMW 320D, part number 64119240180. According to the plastic housing, the device was manufactured by Paragon AG, a company founded in 1988 which has long specialized in the production of these sensors. BMW was one of the first automakers to fit an air quality sensor to its vehicles, first doing so in 1989.

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Taking apart the sensor was no easy feat, as the part designed to survive in the rough and tumble automotive environments. The components inside aren’t just placed inside a plastic housing, they were overmolded with layers of rubbery plastic goop. This serves to protect the internal components from knocks and vibrations, but makes disassembly very difficult for the ardent and curious engineer. In any case, perseverance and a good pair of side-cutters eventually got me a look at what lay inside

In any case, the sensor fundamentally consists of a small sensor element (see below), which lives inside the front of the plastic housing behind the air inlet holes and a simple filter screen. The sensor element is connected to a small electronic printed circuit board, which hosts the components that condition the sensor signal and convert it to something the vehicle’s ECU can readily understand. Depending on the vehicle, Paragon sensors typically communicate with an ECU via PWM signals, LIN bus, or Klimabus connection.

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The sensor element itself is free floating, attached only via wires. This allows it to be easily heated to operating temperature with a minimum of power, as there is minimal heat loss to surrounding components.

The sensor element itself is what actually measures the concentration of certain gases in the air. Just looking at it won’t tell you how it works, but a great deal of research will. Paragon’s most popular sensors detect carbon monoxide, nitrogen oxides, and various hydrocarbons such as those in automotive fuels using metal oxide sensing elements which have the benefit of being able to detect gases in low concentrations. These sensors are also very responsive, meaning they can pick up the presence of a gas quite quickly. They’re known for their good recovery time, which refers to the sensor’s ability to react in a timely fashion when the concentration of a given gas has decreased.

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It all comes down to chemical reactions that happen on the surface of the metal oxide semiconductor sensing element. If you’re confident in your semiconductor science and university-level chemistry, you can dive into the literature, else, enjoy my simplified explanation here.  The metal oxide tends to have oxygen molecules adsorbed, or stuck, on its surface. Those oxygen atoms immobilize electrons in the conduction band in the surface region of the metal oxide. If that sounds too confusing and complex, here’s a simpler explanation: Oxygen on the surface of the sensor locks up some electrons in the metal oxide. This limits the conductivity of the material to a certain baseline level in regular air.

Screenshot 2023 11 29 145345

Left, oxygen molecules becoming “adsorbed”, or stuck on the surface of a metal oxide sensor, trapping electrons. Right, a carbon monoxide molecule claims an oxygen atom, becoming carbon dioxide and freeing a trapped electron. Credit: Metal Oxide Gas Sensors: Sensitivity and Influencing Factors, Wang, Yin, et al. 2010When gases come into contact with the sensing element, they react with the trapped oxygen atoms, and change the conductivity of the sensor. Chemists will tell you that so-called “reducing” gases like carbon monoxide will react with the oxygen atoms to become carbon dioxide. This removes the oxygen atoms from the sensor surface, and their removal leads to an increase in conductivity. This is because the oxygen atoms that are taken away are no longer present to keep the electrons immobilized. By contrast, “oxidizing” gases like nitrogen oxides will effectively donate more oxygen atoms to the sensor surface, immobilizing more electrons and reducing conductivity.

Long story short, certain gases that react with the oxygen on the metal oxide sensor either reduce or increase how conductive the sensor element is. High levels of carbon monoxide, nitrogen oxides, or hydrocarbons can thus be determined by the sensor’s conductivity. When the conductivity shifts up or down from the regular baseline point of clean air, it’s clear there’s an elevated level of the relevant gases that react with the sensor.

The electronics on the sensor measure the conductivity of the sensor element, and process this into a value that can be fed to the vehicle’s HVAC control unit. When elevated levels of the pollutant gases are detected, the HVAC unit shuts the flaps to configure the system for recirculating interior air.

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In many cases, the onboard electronics on the sensor are also responsible for regulating its temperature. The performance and conductivity of the metal oxide sensor element can change with temperature, which could make it difficult to get reliable results regarding the presence or absence of pollutants in different conditions. To avoid this problem, a small heating element and temperature sensor is typically included as part of the sensor to keep the element in its ideal operating temperature range.

 

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These metal oxide gas sensors can be made relatively affordably, which has seen their rollout across more and more vehicles in recent years. Further development in this space is continuing, with companies like Paragon rolling out new types of sensor for picking up particulate pollution and other pollutants. Multi-gas sensors with multiple sensing elements are also becoming more common, using different sensor materials to individually quantify the presence of certain gases.

Historically, these sensors have been a fairly obscure feature. However, their use become more relevant in recent years as customers grow concerned about air quality and its impacts on human health. Notably, Polestar has put the feature front and center in the Polestar 2, with an entire air quality analysis feature available on its infotainment screen, featuring particulate and pollutant analysis.

If you’ve been driving a modern car with a high-end HVAC system, maybe you’ve passively noticed that the dodgy trucks and smoke-belching diesels on the roads don’t seem to smell as bad as they used to. Or, perhaps you never noticed anything at all, as there were no nasty smells to ensnare your nostrils. In any case, it’s worth a tip of the hat to the engineers and scientists that developed these affordable air quality sensors that keep our cars smelling fresh.

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Image credits: Lewin Day, Metal Oxide Gas Sensors: Sensitivity and Influencing Factors, Wang, Yin, et al. 2010

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Stef Schrader
Stef Schrader
7 months ago

Please tell me that someone has tested this against the horrors that are Kansas feedlots.

Canyonero
Canyonero
7 months ago
Reply to  Stef Schrader

The sensor in my VW doesn’t save us from Idaho feedlots, so unless the cows in Kansas are eating something different they probably won’t help there either.

Personally I think that stink is strong enough that it comes straight through the sheet metal.

Stef Schrader
Stef Schrader
7 months ago
Reply to  Canyonero

It’s the absolute worst.

LuzifersLicht
LuzifersLicht
7 months ago

Now that I know that this technology exists, is sensitive enough to give you “an entire air quality analysis” and is apparently affordable(ish) I can’t help but get upset that domestic “smoke” detectors are still as shitty as they are.
I bet one of those sensors won’t mistake me boiling water or having a hot shower for a house fire.

Defenestrator
Defenestrator
7 months ago
Reply to  LuzifersLicht

If you haven’t already, it’s worth trying photoelectric smoke detectors in steamy areas instead of ionizing. They’re a bit less sensitive about that sort of thing.

LuzifersLicht
LuzifersLicht
7 months ago
Reply to  Defenestrator

Thanks, I’ll have to check out what kind I have and possibly badger my landlord into trying the other kind. I like late showers and I’m sure my neighbours are sick and tired of the alarm going off at midnight because I didn’t shut the bathroom door right AGAIN

PL71 Enthusiast
PL71 Enthusiast
7 months ago

This sensor was just chilling not connected to anything in the cowl of a car I no longer have. I now know what it was.

Chronometric
Chronometric
7 months ago

I wonder if my ’76 BMW has this feature. All the button labels are in German and I think one is Funkbegonen.

David Escargot
David Escargot
7 months ago
Reply to  Chronometric

I thought that button switched radio stations….

Chronometric
Chronometric
7 months ago
Reply to  David Escargot

Maybe that’s what happened to my George Clinton cassette.

TOSSABL
TOSSABL
7 months ago

Don’t have anything new enough to have this technology, but it’s certainly interesting. My first question was whether these sensors would react to skunk scent. If I’m understanding this correctly and they work mainly due to certain compounds interacting with oxygen, so I think I understand from Wikipedia that the thiols skunks spray wouldn’t trigger these sensors.

I thought skunks only had one mating season, but that doesn’t seem to be the case in my area

Mike F.
Mike F.
7 months ago
Reply to  TOSSABL

Skunk scent involves mercaptan. I don’t know how they would interact with the sensors offhand, but those sensors would have to be very sensitive to react to them. Our noses pick them up at extremely low concentrations in the air.

TOSSABL
TOSSABL
7 months ago
Reply to  TOSSABL

Funny how the brain processes stuff.
A couple months back I got a strong whiff of skunk. Ran outside and grabbed my little 3-legged mutt. The smell was overwhelming. A few minutes later, while washing him, I realized I couldn’t really smell skunk—rather, it was a vaguely familiar chemical smell. The WP article mentions that there are thiol acetate compounds in there. Didn’t catch it while reading it, but woke up thinking that my sensors must have been overwhelmed and all I could process was the secondary stuff. [shrug emoji]

He’s still a bit ripe when damp, even 4 washes later

Kalieaire
Kalieaire
7 months ago

tbf, in most situations, i find these automated recirc items to be pretty useless.

oftentimes when i come to a stopped position behind someone or something, by the time i hear the system closing up the fresh air vent, i can smell whatever it is it detected. perhaps if it were on the front grill, it might help, but honestly, i think gps based recirculation would help when you know you’ll be traversing cow pastures, etc.

also just manual based with periodic opening for fresh air intake, i think that’s a better option.

also, automated switch to fresh air after the engine shuts off makes more sense.

what doesn’t make sense is that the automatic setting in HVAC turns on automatic fresh/recirc, turn ac to on, and fan speed.

These are three major settings that I want to make changes to independent of the temperature control. I want the thermostat to work exactly as that, a method to adjust the mixing valve to allow more refrigerant or heat laden coolant in.

Comfort wise, sometimes people like having the fan at max, it’s like when people open the window, they like the pressure and the feeling of the wind across the skin, but even so they don’t want to be too hot or too cold.

Hgrunt
Hgrunt
7 months ago

Great deep dive!

When I replaced the AUC sensor on my E46, I went down a rabbit hole reading about what the sensors did and found a whole ecosystem of metal-oxide sensors designed to detect the presence of various gasses

Mine worked after I replaced it, but always seemed a bit slow to respond when behind a diesel truck. I figured it was either due to sensor replacement, hvac computer’s response time, or the threshold to switch to recirculate was on the higher side

It definitely doesn’t detect the presence of cow poop smell though

Stef Schrader
Stef Schrader
7 months ago
Reply to  Hgrunt

Clearly, this should have been invented by a fed-up Midwesterner, not ze Germans.

JShaawbaru
JShaawbaru
7 months ago

Well, now I know why the recirc button in my 2010 335d has 2 modes! I’ve been wondering what the difference was and kept forgetting to look at the manual/search for an answer online.

ProudLuddite
ProudLuddite
7 months ago

So I think this could be very entertaining on a road trip if it were sensitive to gaseous emissions in the car and gave some sort of readout as to the likely source, as well as maybe a scale of how strong the odor was. Think of the fun games and contests that you could play in the car to pass the time on road trips.

Chewcudda
Chewcudda
7 months ago
Reply to  ProudLuddite

Calling it now. The first in-car fart blamer will come from Tesla.

Jesus Chrysler drives a Dodge
Jesus Chrysler drives a Dodge
7 months ago
Reply to  Chewcudda

No, it will be PROMISED by Tesla next year… Really, next year for sure.

Mike F.
Mike F.
7 months ago
Reply to  ProudLuddite

No more Taco Bell on the road trips.

Stef Schrader
Stef Schrader
7 months ago
Reply to  ProudLuddite

IT’S ME. I SMELT IT AND I DEALT IT.

Jim Stark
Jim Stark
7 months ago

Articles like this make me love the Autopian even more.

Glutton for Piëch
Glutton for Piëch
7 months ago

Common thing on V10 TDIs is to get constantly overridden into recirc when in auto, because this sensor is directly above the tandem pump. The gasket for said pump is probably the leakiest on the entire car, so hot diesel drips down the block and smokes.

Ah, VW, my love. <3

Hoonicus
Hoonicus
7 months ago

Firstly Mr Day, I commend you on prodigious article production of excellent quality in a man down situation. Secondly, as greatly appreciated as Jason’s antics are, his disassembly techniques are not to be emulated.
Carry on good man, but consider yourself hitherto-fore banned from reverse engineering any ET technology.

Nic Periton
Nic Periton
7 months ago

I am so glad that my cars rather predate this stuff, other than farts, if there is a strong whiff coming from the car I sort of know how long and how knuckle grazing it will before the smell goes away. The smell of burning leather is not always to do with cars.

Legend of Z3lda
Legend of Z3lda
7 months ago

I also have an i3 with the auto recirculation feature. It’s usually on auto, but I really don’t know how well it works. There are plenty of times I have to switch it to manual recirculation when behind an old pickup or something like that. After that the debate continues whether or not to open the windows once out of the stink zone. It all still feels less than automatic to me.

My 1990 Volvo 240 wagon is easy to figure out. HVAC doesn’t work at all so the windows are down most of the time.

Mike Harrell
Mike Harrell
7 months ago

If any of my cars stopped producing odors I’d know to really start worrying.

Data
Data
7 months ago

I keep recirc on at all times.
In older cars with manual levers, this was not a problem.

In my previous car, a 2008 Honda, using the front defroster would set A/C, recirc, and airflow to the windshield. I always punched recirc back on manually. A/C works fast enough to defrost the window.

In my current Mazda, recirc cannot be overridden when using defrost. I am forced to inhale diesel exhaust, poorly tuned Altima exhaust, the occasional skunk, and the pleasant aroma of a landfill near my place of employment. Thankfully I don’t live near any paper mills.

I want to be back in full control of my HVAC in my next car, but I suppose it’s unlikely.

DadBod
DadBod
7 months ago

How precise are these sensors? Could we identify who dealt it no matter who smelt it?

Sklooner
Sklooner
7 months ago

This summer due to wildfires the Volvo was in recirc all the time

Angrycat Meowmeow
Angrycat Meowmeow
7 months ago

I need one inside the car to automatically roll down the windows when my greyhound farts

Balloondoggle
Balloondoggle
7 months ago

Those are epic farts.

Cheap Bastard
Cheap Bastard
7 months ago

Do not feed that or any dog greasy leftover Thanksgiving turkey before a long car trip.

Trust me on this one.

Fenton Canaby
Fenton Canaby
7 months ago

I’m not sure I’ve ever experienced this in person. Whenever we pass something funky, there’s always the debate as to whether we should turn on the air recirculation (to stop more funk) or keep it off to allow the fresh air in once the funk has passed. I’m in the former camp.

Drew
Drew
7 months ago
Reply to  Fenton Canaby

I’m with you. Recirculate for a few miles, then bring in the fresh air. Don’t keep bringing in more funk in hopes of fresh air.

Glutton for Piëch
Glutton for Piëch
7 months ago
Reply to  Drew

Ever driven through Eastern NC? Made the mistake of rolling the windows down on a drive to see my brother, cause it was ‘such a nice evening’. PIG FARM. Then 200 MILES of pig farm. The A/C on recirc with a brand new charcoal filter couldn’t get rid of it after over an hour. It was wretched.

Last edited 7 months ago by Glutton for Piëch
Drew
Drew
7 months ago

Pig is worse than cow, but marginally better than chicken. I don’t know how they can smell so vile. We even had a fair amount of chickens when I was growing up, and I never got used to the odor.

If you know you’re driving through those sorts of areas, recirculation goes on well ahead of time and stays that way.

Glutton for Piëch
Glutton for Piëch
7 months ago
Reply to  Drew

There were chickens, too, but thankfully far and few between. I had never driven through there, as he had just moved his sailboat to a new marina. We now drive the 2 hour longer interstate route just so we don’t have to deal with it.

It was such blissful ignorance. We had the A/C and recirc on, and thought ahh, let’s open the windows… and our fate was sealed.

DadBod
DadBod
7 months ago
Reply to  Drew

Ugh, getting stuck behind a chicken truck in the summer is so stinky

Fenton Canaby
Fenton Canaby
7 months ago
Reply to  Drew

Printing this to show my wife. Pray for me.

Ranwhenparked
Ranwhenparked
7 months ago
Reply to  Fenton Canaby

I just light up a Camel and fill the car with the rich aroma of fine blended Turkish and Virginian tobaccos

Mike Harrell
Mike Harrell
7 months ago
Reply to  Ranwhenparked

“I’d walk a mile for… most of my cars. And have.”

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