Today: How will driving in circles help Nan pass her emissions test?

Dear Car Talk

Dear Car Talk | Jun 01, 2011

Dear Tom and Ray:

Goofy problem here: We were loaned a Jeep Cherokee by our daughter and son-in-law while our vehicle is being repaired. With a catch, of course: It needed a "speed sensor" in order to pass the emissions test. We got the part, had it installed and promptly failed the emissions test. We were told that we needed to put 50-100 miles on the thing before they could retest the emissions. Is that standard for electronic (or computer-ish) parts? Is there some rationale I'm missing on that? Basically, I need to know so I can tell the judge why I'm driving without an inspection sticker. How are you supposed to put 100 miles on the car in order to get it inspected if you're not supposed to drive it until it passes inspection? Thanks! -- Nan


RAY: Good questions, Nan. But your emissions inspector is right.

TOM: Here's how inspections work these days. A scanner gets plugged into your car's computer port. That scanner downloads a bunch of emissions-related information that your car's computer has monitored and stored.

RAY: But if your battery dies completely, gets disconnected or the computer's memory gets wiped out for some other reason (which I'm guessing is what happened in your case), the computer doesn't have enough stored data to determine whether you should pass inspection.

TOM: So when you drive around for those 100 miles, what the computer is doing is collecting fresh data.

RAY: For instance, the computer looks for engine misfires, which can increase pollution. If there are no misfires after a certain number of miles, it checks off that box. If enough miles go by and your oxygen sensor shows no faults, the computer checks that off the list, etc.

TOM: Then, when you go back for your inspection, the computer can report that it has sufficient data in all areas. And if everything is working correctly, bada bing, you get your sticker.

RAY: If there's not enough data collected, or if everything is NOT working correctly, then you fail your inspection. I don't know how it works in all 50 states, but where we live, you then get a temporary "Rejected" sticker, and you have 60 days to drive around, humiliated, fix the problem and get re-inspected.

TOM: So I'm guessing that your daughter and son-in-law failed inspection a while ago because the vehicle's speed sensor didn't work. And their 60 days had run out by the time they handed you the keys. So when you failed, it was for the second time, and there was no longer a grace period.

RAY: But keep in mind that rejection builds character, Nan. You'll need that character when you're doing your 100 miles on private property, rather than on public roads, to avoid a ticket. I calculate that'll be about 900 times around the local Walmart parking lot.

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