"Check Engine Light" Mystery, It appears only to suddenly disappear... Options

[TheRockman]

OK, here's the deal. This past Sunday while driving home I get the infamous "Check Engine Light". My odometer had just rolled over to 139,149 miles. I mean just clicked into that mileage and the light turns on. SURPRISE!

So I'm thinking, "Oh crap here it goes. Here's where the engine becomes a cranky old man." But this morning, I'm driving to work and the light turns off! Nothing! Again, the odometer had just clicked over to 139,234 when this happend.

So good news: Check engine light is gone. Mystery: Why did it come on in the first place?

I know that my Dad's old Dodge Caravan would turn on the check engine light at about every 50,000 miles (which was crap) and that was the first thing that came to mind about this situation.

I'm going to get the car checked out Friday and see if any codes were stored or if it's still throwing some sort of trouble code without the light.

In the mean time, does anyone have any ideas about what might be going on? Has this happened to anyone?

[DSToyo]

you didnt happen to fill up the gas tank did during that time did you.
-a lot of times if you take the gas cap off and the screw it back on tightly i mean like 20 clicks the light will go off when you start it back up again

[TheRockman]

Nope. The light came on and went off on the same tank of gas. I ran the tank to empty today and refueled earlier this evening. I made sure I turn the cap until it clicked a few times like I always do.

I just think it's an odd coincidence that it was on for almost exactly 85 miles (according to the odometer). But then again, it could have been just that, a coincidence.

[alltracman78]

When the ECU sees a problem, it turns the CEL on.
If the problem is not seen for 3 consecutive trips, it will turn the light off.
It WILL still store the code however.

This is most likely what happened.
You can go by Autocrap or somewhere and have them scan to see if it did store a code, and if so, what it is.
It would be good to know for future reference.

[TheRockman]

When the ECU sees a problem, it turns the CEL on.
If the problem is not seen for 3 consecutive trips, it will turn the light off.
It WILL still store the code however.

This is most likely what happened.
You can go by Autocrap or somewhere and have them scan to see if it did store a code, and if so, what it is.
It would be good to know for future reference.

Sounds reasonable. I'm going to go tomorrow and have the car checked. I'll let you know what happens.

[Davesceli]

OK, here's the deal. This past Sunday while driving home I get the infamous "Check Engine Light". My odometer had just rolled over to 139,149 miles. I mean just clicked into that mileage and the light turns on. SURPRISE!

So I'm thinking, "Oh crap here it goes. Here's where the engine becomes a cranky old man." But this morning, I'm driving to work and the light turns off! Nothing! Again, the odometer had just clicked over to 139,234 when this happend.

So good news: Check engine light is gone. Mystery: Why did it come on in the first place?

I know that my Dad's old Dodge Caravan would turn on the check engine light at about every 50,000 miles (which was crap) and that was the first thing that came to mind about this situation.

I'm going to get the car checked out Friday and see if any codes were stored or if it's still throwing some sort of trouble code without the light.

In the mean time, does anyone have any ideas about what might be going on? Has this happened to anyone?

Mine did that when the pcv went bad. It would go one for a couple of days then turn off. Then come back a few days later.

[TheRockman]

Alrighty, here's the verdict.

Reading the OBDII port resulted in the code PO446 which is an "Evaporative emission control system vent control circuit malfunction." My mechanic said that a few possible causes could have been a loose gas cap, loose hose, or a bad charcoal canister. Since it could have just been a fluke, he cleared the code. If it comes back we may need to investigate further, but for now it's off and I'm happy.

Anyone have suggestions on things I should check?

[alltracman78]

IIRC PO446 is a small leak.
You probably left your gas cap a bit loose.
That or the sealing surface is slightly corroded.

When you tighten the gas cap, ALWAYS make sure it clicks.
And hit up the sealing surface [there's a seal on the cap that seals to the metal ring of the fill tube] with some FINE sandpaper, to clean off any corrosion.

There is also the possibility the canister or a hose, but they are less likely.
If you do both of the above and it still comes on, take a look at the hoses and such.

[TheRockman]

Awesome. Thanks for the advice!

[alltracman78]

You're welcome.

[Agent21]

That or the EGR valve is clogged. I know mine it. Will be working on that tomorrow.

[alltracman78]

That particular code has absolutely nothing to do with the EGR system...
It is an EVAP code, and only concerns the EVAP system.