Car Buff Q: That Emissions Gizmo in New Cars

For the most modern of cars, emissions testing in the tailpipe became passe. Now, there is some computer module the tester hooks a cable to for testing purposes. What is the name of this doohickey module, and how does it work? What model year did this start with?

Is this gizmo periodically self-checking the emissions all the time? Can’t it be wrong? What keeps this hugger in perfect calibration over the life of the car? (Must be some hugger to never need re-calibrating!) And, what happens if this bugger fails? Does one have to throw away a perfectly fine car (or go broke fixing the bugger) just because we’re so stupidly dependent on machines? Can one request an old-fashioned emissions test if that gizmo is fried?

We have boldly gone where no man ever wanted to go.

  • Jinx


And even with ODB II, they still have a tailpipe sniffer, at least in MA. The sensors in various components of the car signal errors, the computer just collects and stores them.

The OBD-II system just checks a ton of sensors out.
Given that OBD-II is operated by a microprocessor, no calibration is required.
From time to time the sensors fail and may need replacement.
Most people would be familiar with the O2 sensor, which appears to be the most common reason you get Check Engine lights in the wild.

The OBD (On-Board Diagnostics) is what people call a “code scanner”. Newer cars use computers to perform engine control, and any anomalies are recorded, and can be read out later through the OBD-II connector. Some of the newer cars have hundreds of different parameters that are measured. The OBD-II system does not actually measure the pollution coming out of the tailpipe - it’s just assumed that if all the other engine parameters are in spec, that the engine is running clean.
Note that the interface is two-way - with the correct equipment, one can actually change performance in some modern engines, often at the expense of pollution.

It DOES kinda’ meter pollution.
It has sensors upstream and downstream of your catalytic converter that can tell if the cat is doing its job. If your reading afterwards isn’t different than your reading BEFORE the cat, it’ll throw a flag. I’m THINKING it uses an O2 sensor for that. Not exactly sure.

Sure, but it’s inferred - There are O2 sensors in the waste stream, and the computer knows how much fuel was injected, and it knows how much air was ingested (mass-flow meter), so it can tune the combustion for optimum efficiency. Still, there are no HC or NOx or CO sensors in the exhaust…

In California (my part, at least - some rural counties might be exempt), they most certainly DO still sniff tailpipe emissions directly as part of a biennial smog check. As in, they physically stick a probe up your car’s cornhole while running it on the dyno.

Your final printout will give you the values they measured for HC, NOx, CO, and perhaps some other things. Your car will fail if any of those things are out of range. ALSO part of the test is a hook-up with the OBD-II computer (on vehicles new enough to be so equipped) - any issues there can cause a failure, as well.

I’ve always had them stick a probe up my car’s tailpipe and run the engine at a certain RPMage for a loooooooooooooooooooooooooooooong time, but just last week I registered a 1998 Ford Escort and all they had to do was connect the emissions station to the OBD port under the steering wheel and they were done in two minutes, no probe necessary. So apparently the capability to do so was there by the 1998 Fords at the latest.

Don’t the sensors need calibration? At least from the getco, the sensor must have been tested against a baseline, right? Otherwise, you have no point of reference. Im not saying the industry would benchtest every sensor, but surely there was some statistical analysis, in the least.

Beyond that, what makes us so sure they cannot go out of calibration?

Sure, the sensors can go out of calibration and the sensors do fail.
When I made my earlier statement, I was saying that the computer can’t go out of calibration, not the sensors. I should have used more qualifiers in that statement.
On the plus side, if the readings from the o2 sensor are consistently reading impossible values, the OBD will throw a check engine light and a code indicating a sensor fault. Nonetheless, minor miscalibration has to occur in the real world.