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I drive a 4 cylinder 2001 Honda Accord LX and recently my check engine light turned on. The code was for the EVAP system, but I was wary of having it repaired immediately. I see 3 problems with a problem in the EVAP system, and they are a slightly reduced fuel efficiency, an inability to pass emission tests that no one tests for, and a check engine light that is on. However, before I could make a decision of whether or not to fix the system, the light turned off. The same thing happened last winter around this time, and the battery was never disconnected in either case.

So I have a few questions:

Could there be a problem with my CEL and the computers that operate it? If not, why did the CEL turn itself off twice now?

Are my assumptions for the problems associated with a failing EVAP system accurate? And if not, what is a more accurate synopsis of the problems?

If those assumptions are accurate, would it probably be cheaper in the long run to simply turn off the codes relating to EVAP than actually repairing it? A small leak of gasoline vapor over a 5 year period doesnt seem like it would be more expensive than a couple hundred of dollars in parts and labor.

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What was the exact code? There are multiple possibilities for the EVAP system. –  Mark Johnson Jan 10 '13 at 20:36
    
Yeah I know there are, unfortunately the shop i took it to didnt write the codes on the parts estimate form and the exact time I went to drive to Autozone to get the code read is when the light turned off on its own. If it turns on again I will get it checked right away. –  lakechfoma Jan 10 '13 at 22:16
    
Even if the light is off, there might still be a stored code. Might be worth checking. –  Mark Johnson Jan 10 '13 at 22:21
    
The code should still be stored if you can get it read soon enough. It won't be cleared until a given number of good test cycles have occured. –  mikes Jan 10 '13 at 22:22
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2 Answers 2

up vote 2 down vote accepted

The cars' computer runs tests on the emission systems on the car. When the car fails the test it turns on the light, if it passes the test after that it turns off the light. You may have something that is close to failing the parameters set by the OEM. A small leak that's just on the edge of passing. Something like temperature changes, or vibration could cause it to fail only sometimes. In the case of the EVAP system you may have left the gas cap loose which would have set a code. The next time you filled up and tighten the cap and the computer ran the test and it passed it would have turned out the light.

As for you other question you can't just turn off the codes, if it fails you will have to get it repaired in order to turn out the light. Now depending on the inspection laws where you are at you could just ride around with the light on and not worry about it if you don't want to fix it. If it's required for inspection you will have to repair it or get a waiver to pass inspection

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Now I had heard the gas cap theory before as well, but that doesnt fit because i hadnt gotten gas between when it was on and turned itself off. I thought the light would only turn off if it were reset with one of those car computer diagnostic tools, hence being confused. Also, I know its probably frowned upon if not illegal to manually reset the code with a scanner but...is it possible? It seems like the cheapest solution to a car that will be scrapped in a few years. –  lakechfoma Jan 10 '13 at 15:24
    
@lakechfoma resetting the code will only turn the light off until the next time if fails the test. The car is actively testing the emissions system on a regular bases. –  Larry Jan 10 '13 at 15:56
    
Figured as much but thought I would try asking around a bit more anyways. Thanks for the help though. –  lakechfoma Jan 10 '13 at 16:00
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I doubt anything is wrong with your powertrain controller. It sounds like it's doing what it's supposed to, monitoring the EVAP system and throwing codes when it sees a problem. The problem went away or the check engine light would not have gone off. Some trouble codes can be self resolved like that (so many drive cycles without a problem, the light goes out), others are more permanent (light stays on until reset). Either way, there is still a stored trouble code waiting to be pulled whether the light is on or not (unless the battery is disconnected).

As for the impact of an EVAP leak, reduced fuel economy sounds plausible. Most modern emissions tests, where required, are moving towards simply connecting to the diagnostic port and confirming that the emissions monitoring is occurring and there are no trouble codes. The check engine light being on is an automatic fail just about everywhere. A stored code, whether or not the light is on, may be a fail depending on the locality and how extensively they test. Modifying your powertrain controller to not monitor the EVAP system is not going to be easy, and I doubt it's legal anyway. You might not get caught, but the risk/reward profile does not make sense.

Here's the real reason you want to fix the problem, though. If you get used to having that light on and ignoring it, you're going to miss a real problem. Unless you plug in a diagnostic tool periodically to confirm no new codes have appeared. It's a lot easier to just look for a leak in a rubber hose and fix it.

I've had a couple EVAP leaks fixed by a shop. One I wasn't even charged for, they just replaced or patched a short piece of hose (I was paying for other work, though). One involved replacing a valve near the fuel tank. The latter one was a couple hundred bucks in parts and labor. That's probably the worst case as long as parts are available.

Get that code pulled and see what the problem is. If it's just a simple leak, you might be able to fix it yourself for next to nothing.

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I was told by Autozone, Goodyear, and a car geek friend of mine that if the CEL is no longer on the code can't be pulled? My thought is that the CEL being permanently on is the biggest problem if I choose not to fix it, for the exact reasons you outlined. I have a local independent repair man that I should probably contact at this point. –  lakechfoma Jan 10 '13 at 22:22
    
I thought there would be a stored code present even if the CEL was off, but I could be wrong. If it comes back on, get it pulled if it stays on long enough. Code readers are pretty cheap nowadays and they're handy to have around. –  Mark Johnson Jan 11 '13 at 1:06
    
At this point I really should just get one so I can check it myself and any future incidents. Most look to be under $100 and can probably pay for themselves in the right circumstances. –  lakechfoma Jan 11 '13 at 1:28
    
@lakechfoma All OBD II cars have history codes. Some of the less expensive code readers (like the one Autozone uses) will only read current codes. Any professional repair shop will be able to read history codes and freeze frame data (certain values stored at the time of the failure). –  Larry Jan 11 '13 at 5:43
    
If you have a laptop, Android tablet/phone, or even an iThingy, you might want to pick up a USB (for the laptop) or Bluetooth ODBII adapter. The cheap ones are < $20 on Amazon. There are various software packages to use the adapters for Windows, Mac, Android, and iOS. –  jwernerny Jan 11 '13 at 14:05
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