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Clarence Martin
12-13-2014, 11:10 PM
Got a question for any auto mechanics on the forum. Had the vehicle inspected. Problem is, it's my Mom 's car and she is unable to drive it at the present time. She was supposed to drive it 100 miles as per the mechanic, he said he cleared the computer and needs to check it again after 100 miles. If that car gets taken back to the mechanic before 100 miles is put on the car, will anything show up on the computer to cause it to fail the computer test ?

Wade Lippman
12-13-2014, 11:19 PM
Presumably he "fixed" something and cleared the computer. He needs give it a chance to fault again to see if he got it. If it shows up again he will have to try something else.
I have had a problem like that with the fuel cap on a couple cars.

Greg Peterson
12-13-2014, 11:19 PM
Depending on the make/model/year of the vehicle and the nature of the code, the vehicle will need to go through several drive cycles to clear the code(s). I think once a code is set, the system needs to see a certain number of cycles without to error code being thrown to make sure the problem has been fixed.

Tom Stenzel
12-14-2014, 12:45 AM
If the mechanic cleared the computer then whatever error was in it is now gone. I'm hoping he had a reason to reset the computer. But when the computer is reset to clear a diagnostic fault it also resets the OBD II monitors. These monitors are indicators that diagnostics have run to completion and indicate all is OK with the engine and emission controls. They can be checked with a code reader to see if they've completed. The car will have to be driven enough to run through those diagnostics.

The 100 miles is not a fixed number. If you drove the car on 50 two mile trips the diagnostic programs won't have completed and the monitors show not ready. What is important is driving cycles. Starting with a cold engine and driving it until it is fully warmed up is a driving cycle.

The are a few gotchas, a broken gas gauge is now effectively against the law as the evaporative tests won't complete without the gas gauge reading between 25% to 75% (approximately). Keep the gas tank full all the time? You sorry so and so!- the car won't pass.

My Saturn gets close to 35 MPG. If I filled the tank up I would drive close to 100 miles - and maybe a few more - before the evaporative diagnostic would even begin. The gas gauge in my wife's van has been broken for two years now. We got the cost of repairing it- it'll get fixed if the fuel pump goes kaput. A gas gauge is nice but it's not a necessity. We've lived without.

Try googling "OBD II monitors", "OBD2 monitors" or something of the sort to see what the fuss is. My reading is that the monitors should be set in less than 100 miles as long as you make sure that there's no problems. I think the mechanic is trying to make SURE all the tests are completed with his 100 mile statement.

BTW: The check engine light comes on if there's a fault. It's not on while the diagnostics are running but not completed.

-Tom

Ty Williams
12-14-2014, 3:35 AM
Got a question for any auto mechanics on the forum. Had the vehicle inspected. Problem is, it's my Mom 's car and she is unable to drive it at the present time. She was supposed to drive it 100 miles as per the mechanic, he said he cleared the computer and needs to check it again after 100 miles. If that car gets taken back to the mechanic before 100 miles is put on the car, will anything show up on the computer to cause it to fail the computer test ?
I am a professional mechanic specializing in Subarus.




Depends on the exact laws in your state.

Your state is plugging a computer into your car and asking it if it thinks it might have any problems. The computer can reply in 3 ways:

1) Yes, I have a problem (or multiple problems) and here's the category of problem I have.
2) I do not yet have enough data to know if I have a problem or not.
3) I am confident I do not have a problem.

In all states that inspect, #1 is a failure. In some states, #2 is a failure and some states allow you to slide by even with #2. In all states, #3 is the answer they're really looking for.

So, what's the difference and why did the mechanic want you to drive 100mi?

With the government mandating OBD-II, the car is constantly running a bunch of tests on itself so it can make an educated guess if it's meeting emissions requirements or not. These tests require you to do specific things as you drive the car in order for the tests to run. Once the test has run, the car can mark itself as "Yes, I've completed this test successfully at least once" or "No, I failed this test.". If you have not driven the car in the ways required for the tests to run, the car will mark itself as "I haven't yet been able to complete all the tests.". If the test does run successfully, the car marks itself as passed and then immediately starts looking for the opportunity to run that test again. Basically, the car constantly runs and re-runs these tests over its whole life so that it's constantly monitoring its own behavior.

So here's how this impacts you:

When the computer in the car first wakes up (which can be after things like having the ECU reset, having the codes cleared, or having the battery detached for a moment), it hasn't run any of the tests and will report "Not Ready" on all the tests. As you drive the car around, you will accidentally do the right kind of driving to allow each individual test to run. When the car successfully completes that test, it marks that test as "Ready". If the car fails a test, it sets a Diagnostic Trouble Code (DTC) that indicates what test it failed (and in VERY general terms why), turns the Check Engine Light (CEL) on, and typically disables the cruise control and flashes the cruise control light (on cars newer than about 2005 or so). What happened to your mom's car is that the mechanic thinks he fixed something. He cleared the codes so the car would stop complaining about whatever it was he fixed. Now all the tests are reporting "Not Ready". In order for your car to pass inspection, all of those tests need to become "Ready" by you driving the correct pattern to allow the test to evaluate. He said drive it 100 miles because he figures that you'll probably accidentally hit all the right things within that 100 miles and all the tests will then report "Ready" and you can get your car passed at inspection.

Now, here's the thing, the 100mi is just a guess. You could drive 1000mi and never do the right thing. You could drive about 5mi and get all the tests evaluated. It all depends on whether you accidentally do what the car wants to see in order to run the test. It doesn't matter how far you drive, it doesn't matter how many times you get the car hot and cold, it doesn't matter how many times you turn the engine off and back on. It all comes down to getting specific patterns of driving. The patterns the car is looking for are called drive cycles. I'm quoting from the factory service manual for a 2006 Subaru Impreza 2.5i here to give you some examples of drive cycles.

Drive Cycle A: After running for greater than 20 minutes at greater than 50MPH, let engine idle for 60 seconds.

Drive Cycle B: Drive the vehicle at 6MPH or greater and then let engine idle for 10 minutes.

Drive Cycle C:
1) Idle engine for 60 seconds
2) Accelerate to 60MPH in less than 20 seconds
3) Hold speed at 60MPH for 20 seconds
4) Decelerate with fully closed throttle down to 40MPH
5) Hold speed at 40MPH for 10 seconds
6) Accelerate to 60MPH in less than 10 seconds
7) Decelerate to stop with throttle fully closed


The list goes on for a total of 6 different drive cycles and all have requirements for fuel level and engine temperature as well.


So, as you can see, the 100mi thing is just a guess. Just go out and drive the car like a normal person and eventually all the tests will run.

John Huds0n
12-14-2014, 3:53 AM
I have a 2002 S10 pickup that does not get driven very much. Battery died and it needed a smog test. Installed a new battery and drove to the station - failed the test for reasons mentioned above.

Hopped on the freeway drove 20 miles down and another 20 back to the smog station and it passed

Malcolm Schweizer
12-14-2014, 6:55 AM
So in the states they are now plugging in to the OBDII for vehicle inspection??? Wow. We pull up, the guy checks the lights, sometimes has you set the e-brake, and sends you on your way. If you fail, slip him $20 and you pass. In fact, there are guys that for $75 will go register your car for you (well worth it because lines are long and it takes 3 hours). These guys are authorized to do the inspection for the BMV. Of course they just sign off and never look at it.

Ty Williams
12-14-2014, 12:19 PM
So in the states they are now plugging in to the OBDII for vehicle inspection???
No, I meant "all" as in "all states that have inspection".

Here in Ohio, we have absolutely no sort of inspection of any kind. You pay them the money, you can get anything, no matter how bad of condition, licensed and stickered.

Jim O'Dell
12-14-2014, 12:38 PM
I've seen cars clear the monitors in 30 miles, and others not clear them after 300. I agree the 100 miles is a "rule of thumb". I've also seen the computer in the car needs to register accelerations, decelerations, key on/off, engine temp to running condition then cool off. I tell customers to just do a few days of "normal" driving...to and from work, to the store, etc. The state inspection computer here in Texas will pass if one or maybe two certain monitors have not cleared, but emission monitors have to clear no matter what. They should be able to plug in a diagnostic computer and check if all the monitors have cleared and if not, which ones still need to clear before putting the car back on the inspection computer. Jim.

Jerome Stanek
12-14-2014, 4:44 PM
No, I meant "all" as in "all states that have inspection".

Here in Ohio, we have absolutely no sort of inspection of any kind. You pay them the money, you can get anything, no matter how bad of condition, licensed and stickered.

Maybe the county you live in but here in Medina you have to have an E check.

Ty Williams
12-14-2014, 5:39 PM
Maybe the county you live in but here in Medina you have to have an E check.
Only two counties in Ohio do that. The other 86 counties do nothing at all.

On average, Ohio doesn't inspect.

George Bokros
12-14-2014, 5:52 PM
Only two counties in Ohio do that. The other 86 counties do nothing at all.

On average, Ohio doesn't inspect.

Not true. In Ohio the following seven counties have mandatory e-check --Cuyahoga, Medina, Summit, Portage, Lake, Lorain, Geauga. The other 81 do not.

Myk Rian
12-14-2014, 6:26 PM
Who cares about Ohio? Has nothing to do with the OP.

Ty Williams
12-14-2014, 8:02 PM
Not true. In Ohio the following seven counties have mandatory e-check --Cuyahoga, Medina, Summit, Portage, Lake, Lorain, Geauga. The other 81 do not.

Huh, that's risen then. Last time I had a guy who had to care, it was just two counties up near Cleveland. Still, 7 counties are doing it, 81 aren't. You guys can safely be ignored :lol:

Matt Meiser
12-14-2014, 10:38 PM
A mere 1/4 of the population. :rolleyes:

We had a code on my wife's car that most likely could have either been a dirty seal on the capless fuel filler or a bad $15 valve that took 10 minutes to change including finding the right socket. I spent a lot more time on it though as it took about a week of driving to meet the requirements to turn the light back on after I cleaned the filler, then about 2 weeks to clear the permanent code after replacing the sensor. At that time she was driving 25-50 miles a day.

Erik Loza
12-15-2014, 12:53 AM
What's the actual code in question?

10 years experience working on OBDII vehicles, by the way...

Erik Loza
Minimax USA

Ole Anderson
12-15-2014, 9:52 AM
This thread makes me happy that MI doesn't require testing anymore. They did tailpipe emission tests for a few years back in the 80's. One of the advantages to living in "Pure" Michigan!:)

Erik Loza
12-15-2014, 10:28 AM
I'm going to out on a limb and take a stab at it: Code P0420? "Catalyst below efficiency"? That would be the common one that us shade tree mechanics clear by disconnecting the battery (or pulling the ECU fuse). The car will not display the CEL for a number of miles (perhaps 100) but the issue is that it still won't pass OBDII inspection. Once the tech plugs the scanner into the car's port, the ECU will report "not ready", which is why they tell you to drive it around for a while.

There are some aftermarket fixes that might be work and be inexpensive if this is the code in question.

Erik Loza
Minimax USA

ken masoumi
12-15-2014, 12:08 PM
This thread makes me happy that MI doesn't require testing anymore. They did tailpipe emission tests for a few years back in the 80's. One of the advantages to living in "Pure" Michigan!:)
I wish they would put an end to this "drive clean program"that we have up here in Ontario,which is probably very similar to the one in California,95% of the cars pass with flying colors,cars that mostly fail were built between 1980 to 1990,and largely as " the result of system malfunctions, not increased vehicle emissions."but Unbelievably, all vehicles built before 1987 are exempt from Drive Clean testing!!

George Bokros
12-15-2014, 1:52 PM
Here is one for you. I have a hobby car - Mercury Capri RS 302 V8 like the Mustang GT. It has a supposedly (per Ford Motor) non emissions legal cam in it, a re-jeted (larger jets) 4bbl carb. In 2000 it tested cleaner than the Ranger I was using as my daily driver. It is now exempt since it is over 25 yrs old.