In the VW emissions cheating scandal, news reports say the following:

  • According to the EPA, "the position of the steering wheel, vehicle speed, the duration of the engine's operation, and barometric pressure" (all very specific indicators of an emissions test) acted as the activation switch for the "defeat device." Essentially, the vehicles' electronic control module (ECM) was set to "clean" mode for the remainder of the emission's test procedure.


How is barometric pressure an indication that a test is being performed?

Is the test performed in a low-pressure chamber or something?

  • The test isn't performed in a chamber. Barometric pressure has an effect on fueling (how much fuel the car's computer needs to inject to mix correctly with the amount of air sucked in by the engine). Too much fuel will lead to unburned fuel in the exhaust (more emissions). I don't see how barometric pressure could be of any indication that the car is being tested.
    – Shamtam
    Sep 25, 2015 at 20:11
  • I don't get that, nor do I get how steering wheel position matters for a test either. Do you have a link to where the quote is from? I'm interested in reading the entire story (as they wrote it) so I can see if there is some context there. Sep 25, 2015 at 21:04
  • @Paulster2 I'm guessing the position of the steering wheel was factored in because a car is stationary while an emissions test is being done. The steering wheel would be moving, even minutely on a straight road, in standard driving conditions. Sep 25, 2015 at 21:31
  • In my state (Colorado), emissions tests are performed on a "treadmill" with the car in gear and the wheels spinning at a certain speed. A fan is placed in front of the car to help cool it down during this process and a cone is just stuck behind the car to capture the exhaust. The only way I can think of that a car could detect this situation is if the wheels are spinning at, say, 35 mph and the wind is coming in at 20 mph and the car can detect that. Barometric pressure depends on wind speed, so this could be detected theoretically. This is just speculation, so I won't answer the question yet. Sep 25, 2015 at 21:42
  • @PoissonFish - Are you sure OBDII cars are ran on a treadmill? According to this source only pre-OBDII cars would be run on a treadmill (dynomometer) which would exclude the new VW's in question. My state doesn't test OBDII vehicles on a treadmill, either. Sep 25, 2015 at 22:08

2 Answers 2


The federal test procedures for vehicle emission certification(FTP) is run on a dynamometer. It starts with the vehicle at an exact temperature and barometric pressure and humidity. It is run in a lab that has carefully controlled exact air conditions that are maintained throughout the test. The vehicle is kept is stored at the test temperature for hours before the test so that it stable. It is lifted onto the dynamometer, the engine is not started until the test is begun. The steering wheel is not moved during the test. It is driven on a very exact speed, load and time profile while the tailpipe gas emissions are measured. It would be simple for the onboard computer to recognize the beginning of the test and put the computer in a clean drive mode. The exact temperature and barometric pressure would be enough to start the computer to looking for the test. A picture of the test profile:

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  • Ah, if the temperature and barometric pressure are EXACTLY specified as part of the published test procedure, I can see how the car could use that as one of the things to detect that it was being tested. ... For those who wanted a link to the source, all of the news stories I read (New York Times, Los Angeles Times) used the quote that I listed above. The violation notice is at www3.epa.gov/otaq/cert/documents/vw-nov-caa-09-18-15.pdf. ... I do understand the steering wheel thing which someone asked about. The car is not being driven; the steering wheel and rear wheels don't move. Sep 28, 2015 at 14:43

In my home state of Colorado, we are required to get emissions tests on our vehicles every two years (after they are a certain number of years old). They way they do this is they place the car on a dynamometer (like a treadmill), put the car in gear, and bring the wheels up to a certain speed. They have a fan placed in front of the car to help cool it down during this process and a cone behind the car captures the exhaust and measures it.

The way I see it, a car could detect it is being tested for emissions through a combination of the following:

  1. The steering wheel is stationary. During normal driving conditions, even on a completely straight, flat road, there will be at least minute changes in the steering wheel. The car could detect that the wheels are spinning, but there is no change to the steering wheel.

  2. I don't know about the duration of the engine's operation, but the vehicle speed and the load can certainly be detected to some extend. If the wheels are brought up to 35 mph but the car is not moving, there must certainly be ways for the car to determine that it is not having to work as hard to maintain speed as it would if the car were actually in motion. This could be negated by adding load to the dyno.

  3. Wheel speed vs. actual speed. Most cars manufactured today have a GPS unit, which can detect speed (much more accurately than your speedometer, I might add). If the wheels are spinning at 35 mpg, but the car is stationary, the car could detect this.

  4. Given that the wheels are turning at 35 mph, but the wind speed from the fan in front of the car is probably different, let's say 20 mph, the barometric pressure will vary from what the car is expecting. Perhaps it is measuring the pressure of the air entering the intake (using the mass flow sensor), or maybe it has a separate barometric pressure sensor for whatever reason, but wind speed plays a factor in pressure. Think of an airplane wing where the wind speed above the wing is faster than below the wing. This creates a lower pressure above the wing, which produces lift. The car might be able to detect that the barometric pressure is not what it would expect from a car going 35 mph. Along the same lines, since the fan is placed in front of the car before they accelerate the wheels, the car could detect that the barometric pressure did not change from 0 mph to 35.

Without reading the article or knowing more information about how VW did this, my response is largely speculative.

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