60 miles of driving before smog test?
February 23, 2015 7:23 PM   Subscribe

Today I took my car in for a smog test. Apparently it was passing all of the steps, but one of the final ones (maybe the final one?) kept malfunctioning.

The mechanic asked if I had either jumped the car or changed the battery in the past few days (I have not). In the end, he suggested that I drive the car for 60-70 miles tomorrow, before bringing it back to test again, and that should fix the problem.

Does anyone know what this should do? What problem is it solving? I've read about driving a car for 20 minutes before a smog test, but 60-70 miles is...much more than that. The mechanic talked about it a little, but I know next to nothing about cars, so it all went right over my head, and now I can't even remember enough of what he said to google key words. Also, do the 60 miles all need to be in a row? (If I stop after 30 and turn my car off to go to the store, will I need to start over?) And he said it should be "normal driving"--does this mean no freeways? Clearly these are questions I should have asked while there, but I didn't think of them until I got home.

posted by tan_coul to Travel & Transportation (6 answers total)
It's some sort of computer reset afaik. I had something like that happen after I had replaced a battery. I don't drive my car much so I hadn't driven it the required amount of miles for the computer thing to reset. I was told that it was cumulative miles. Did that and the car passed no problem.
posted by mandymanwasregistered at 7:49 PM on February 23, 2015

He wants you to show up with a fully charged battery. Every time you start the car, you drain the battery a bit, and then the subsequent driving slowly recharges it (via the alternator being driving by the gas engine's accessory belt). Lots of short drives (lots of starts, little actual driving) can drain the battery, in theory at least.

Assuming I'm right about why, you should definitely not chop it up into smaller runs. However, you likely don't really need to drive 60-70 miles -- I would think 30 minutes would be plenty. Any kind of driving is fine.
posted by intermod at 7:58 PM on February 23, 2015

Has your check engine light been on recently? Because it sounds like you'd be trying to get the code to clear. If that's the case, I don't think they have to be all at once. But you might want to Google about clearing the error codes. I think there are recommended methods to your driving patterns to make sure it resets? I only know what I vaguely remember reading on an Ask here a few months(?) ago, but I don't remember enough to find it.
posted by Weeping_angel at 8:06 PM on February 23, 2015

I had the same problem, though my smog guy said I needed to drive several hundred miles. The problem for me was that I had recently taken my car in for service and whatever work they did required resetting the car's computer. So when I went to get smogged, the computer didn't have enough of the data it needed (gathered from regular driving) to finish the test. I basically drove around all day and went back the following day to finish the test.
posted by cecic at 8:12 PM on February 23, 2015 [1 favorite]

Drive cycle. Google it.

Basically when a car gets a check engine light, a code for some failure is thrown. Once your mechanic fixes that problem (or it's transient like a loose gas cap, and goes away on its own), the computer needs to make sure all the systems are running well. The tests for the various systems are called readiness monitors.

Before the tests can be run, the car needs to be cold. 'Cold' is defined as the coolant temperature sensor reading a value within 11F of the inlet air temperature. Some tests run immediately when you start the cold car. However you can't just start the car and drive off impatiently.

Start the car, let it idle for ten seconds, turn on the a/c (yes I know) and the rear defroster. Foot on brake, put it in drive. Sit there for two minutes at least.

Drive normally to the nearest freeway. Make sure you stop completely and wait for a second at any stop signs along the way. On the freeway, accelerate moderately to between 55 and 60 mph and hold that speed for five-ten minutes.

Coast to an exit, using your brake as little as possible. Come to a complete stop. Head back the way you came, only get on the gas pretty hard (3/4) throttle) when accelerating to 55-60 mph.

Wait eight hours or overnight, then do it all again. Some readiness monitors rely on data from two consecutive cold starts.
posted by notsnot at 5:51 AM on February 24, 2015 [1 favorite]

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