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Some simple questions.

Does the engine malfunction light turn off automatically if the problem gets fixed? As in, if it goes on because my gas cap isn't seated properly, will the light go off when I secure it? Or does the light have to get reset even when the problem is fixed?

If the former, I understand you can reset the light by disconnecting the battery (or pulling the right fuse). What exactly does that accomplish? If the problem still exists after you do that, won't the light come go back on when you power the car back up?

 
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yes....over time it will,

it may take you starting the car, driving it, and turning it off a few times,

the easier way is to just disconnect your battery for 15-20 mins and re-connect

 
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Thank you.

So why do so many people ask about manually turning off that light? If the problem gets fixed, and the light turns off by itself, you don't need to do that manually. On the other hand, if you manually turn off the light when the problem hasn't been fixed, won't it just go on again?

Seems kind of odd to have to power off the car for 20 minutes plus in order to reset that light. Is there some system capacitance that has to bleed off?

 
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Depending on the fault, it could take days of driving before the CEL turns itself off......provided you actually fixed the problem. If you didn't get the problem fixed, how long is it gonna take before you start wondering if it's gonna reset itself or not?
How long before you know for sure you didn't fix it?

That's wasted valuable time. Erase the code when you do your work so the PCM will give a fresh clean slate, then you will know very soon if the problem is fixed...or not.

If you paid someone to fix your check engine light---- and you picked up the car and found the check engine light was still lit up like it was when you dropped it off, would you accept your own answer??? ("The light should turn itself off eventually IF I actually fixed it, but we won't know if I fixed it until the light goes out.")



Seems pretty freekin handy to me. Most other manufacturers require a scanner of some sort to erase codes. Honda lets you DIY it for free.

You don't really even need to disconnect the battery, you could instead remove power source circuits from the PCM to erase memory.

 
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I guess that makes sense. Though it's a bit odd that it can take the car many hours of driving to sense that the problem is fixed. That is, you've either got a bad sensor reading or you don't. If I buckle my seat belt, it doesn't take many hours of driving for the alarm light on my dashboard to go off.

Yes, that is nice that you can reset the system and turn off the light by removing power. I'm just surprised that you need to remove power for 15-20 minutes. There must be some residual capacitance in the system that is holding charge, or else Honda specifically doesn't want a quick power failure to do that reset.

Thank you.

 
Rep Power: 208

it may not necessarily take that long,

that is just what i was personally recommending,

im not understanding what your problem is, or why this seems to be so complicated for you, when its so very simple,

fix the problem you have, wait for the light to turn off, or reset it yourself as was explained.....why does that seem so complicated for you?



.

 
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It can take a while for the light to turn off because the computer runs tests on the sensors. If they pass the tests then everything is considered "good" and the light doesn't come on. If something fails a test then the light comes on. The computer only runs tests durning certain conditions. For instance if the fuel level is really low or really high it wont run an evap test. Whereas an o2 sensor is constantly tested so o2 sensor codes pop up and go away much quicker

 
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Read up on OBD2 readiness monitors, and OBD2 drive cycles.

 
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That's interesting that the computer only runs tests on some sensors occasionally. I guess that has to mean that those sensors it is monitoring occasionally are not seriously related to the health and well-being of the car.But it is rumored that if that engine light blinks, BAD things are afoot, and you need to get service asap. But maybe it does check for those particular things frequently?

A microchip computer can monitor loads of stuff very quickly. Thousands of times a second. You have to program it specially to make it NOT monitor some things some times.

The evap test isn't run if the fuel level is high or low? Interesting. So if I don't seat my fuel cap properly after doing a fill-up, it won't tell me about for a while? And if I fill my tank and reseat it, the alarm light won't go off for a while? Weird.

Why is it complicated for me? Because it just appears to be complicated. If it were simple, my questions would have obvious answers. It's a WARNING LIGHT, and it seems to me that warnings are worth taking seriously. But I appreciate the information.

I did read up on OBD2 monitors. Interesting! It involves what is called an "OBD drive cycle". The answer is that the light may go out when the problem doesn't reappear, and it might not! Yep, built in weirdness. You'd like to believe that Honda has a reason for building in this weirdness. Thank you! A very nice writeup here ...
http://www.autotap.com/techlibrary/o...ns_testing.asp

 
Rep Power: 517

It does get really simple, once you take the time to learn and understand the whys and hows. It's not a black art nor witchcraft.

It's not a rumor. Flashing CEL while the engine is running means the computer detected an engine misfire that could harm the catalytic converter.

 
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Well, darn. I was hoping it was black art or witchcraft. Somehow, that seemed more appropriate.

Still not simple, though. The computer that is generating the fault light is not responding to the lack of fault immediately, and turning off the light. For some reason, for some faults, it's waiting. Maybe until the magic wand is waved? Or until it counts a specific (and no one wants to specify) number of OBD drive cycles? Or until power is pulled for fifteen minutes?

If someone can spell out which faults are monitored when, and when exactly the fault light is turned off, it will be at least a step towards simplicity.

If there is a fault on my computer, I can often cure it by pulling the plug and pushing it back in. I don't need to wait fifteen minutes. 899, 898, 897, 896 .... sigh.

"Simple" is where faults are polled for by the microprocessor a few hundred times a second. Easy peasy. The light comes on when there is fault, and it goes off when it isn't. Hey, that's simple! But that's not the way it is. Not by a long shot. Yes, it would be nice to learn the hows and whys, but I'm only getting fragments of those here.

Thanks for your assistance.

 
Rep Power: 517

Details can usually be found in factory service information.



Each fault code has very specific set criteria, and clear criteria. Each is specific.
Sensors are not only tested in a static condition, they are tested dynamically. This means tested during actual operation while running and driving.
Sensors are used to judge other components or systems that cannot be directly monitored, such as the efficiency of the catalytic converter, and the flow of the EGR system. The EVAP system operates using engine vacuum, so it cannot be tested without the engine running, and there are very specific criteria that must be met in order to get the EVAP test to begin.
There are others too.
There is no way for the computer to perform many of these tests without running/driving the car within a very specific set of conditions.
Many self tests require more than one failure in a row within a specific time or condition period as a precaution against false failures.

So when it comes to getting the computer to extinguish the light on its own, it must first run the test(s) for the code(s) that previously failed, and the system must pass the test(s), and sometimes not only pass the test but pass the test more than one time.
As already mentioned, it's not necessarily 15 minutes.
Hell, if you have the necessary equipment erasing codes may be as simple as a push of a button.
You completely unplug your PC when a simple restart will usually suffice? Weird.


If Microsoft built cars.....

1. For no reason whatsoever, your car would crash twice a day.
2. Every time they repainted the lines in the road, you would have to buy a new car.
3. Occasionally your car would die on the freeway for no reason. You would have to pull to the side of the road, close all of the windows, shut off the car, restart it, and reopen the windows before you could continue.
For some reason you would simply accept this.
4. Occasionally, executing a maneuver such as a left turn would cause your car to shut down and refuse to restart, in which case you would have to reinstall the engine.
5. Macintosh would make a car that was powered by the sun, was reliable, five times as fast and twice as easy to drive - but would run on only five percent of the roads.
6. The oil, water temperature, and alternator warning lights would all be replaced by a single "This Car Has Performed An Illegal Operation" warning light.
7. The airbag system would ask "Are you sure?" before deploying.
8. Occasionally, for no reason whatsoever, your car would lock you out and refuse to let you in until you simultaneously lifted the door handle, turned the key and grabbed hold of the radio antenna.
9. Every time a new car was introduced car buyers would have to learn how to drive all over again because none of the controls would operate in the same manner as the old car.

 
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Thank you ezone. I think you confirmed the complexity of all this.

I have no problem with complexity. I was just pointing out that it sure isn't simple, and I have to believe there are good reasons that it isn't simple. BUT IT ISN'T SIMPLE.

It requires a specific set of criteria, and details for those criteria aren't commonly available, but rather in the factory service information, which, so far, no one here has been able to quote. Oh, if I have the equipment, I can get rid of the indicator alarm at the push of a button. Wonderful! And then the alarm may or may not come back. Less wonderful.

Exactly right that if Microsoft designed cars, cars would crash twice a day. And they'd also be seriously hacked by folks in China and India.

But no, if Apple designed cars, the alarm wouldn't be a non-informative light that stays on for strange durations, and went off after strange durations. That would be a poor user interface, and Apple doesn't do that. The car would be more expensive, but it would hold your hand at every step. And no, you couldn't look under the hood.

My question is one that gets asked a lot, and I've *never* seen a clear answer. Now I understand that a clear answer only exists in documents that most people don't have access to. But at least I have some handwavey answers, and that does help.

 
Rep Power: 517

You can pay subscription fee to Honda and have access to the same information resources a dealer tech has.

You can pay a subscription fee to any of several different information services and get access to the same information resources an independent shop tech may have.