My first "Check Engine Light"

[Sim]

In the morning, I started Gen and light popped up. So I went to a dealer. After 30 minutes of waiting, mechanic told me that fuel cap was not fully closed. Odd - I thought I closed it and did few 'click click click'. In the end, I was charged zero dollars and got a free car wash.

So here's a public announcement - Verify that fuel cap is tightly sealed.

 

[Duaine]

In the morning, I started Gen and light popped up. So I went to a dealer. After 30 minutes of waiting, mechanic told me that fuel cap was not fully closed. Odd - I thought I closed it and did few 'click click click'. In the end, I was charged zero dollars and got a free car wash.

So here's a public announcement - Verify that fuel cap is tightly sealed.

I also might add that you can't keep topping the tank off or the check engine light comes on. In other words if you try and top off the fuel when you have anything more than 3/4 of a tank already the light will come on. It happend to me both times I tried it because I was trying to top off before trips both times.

My dealer checked with Hyundai and for whatever reason the fuel level sensors and the computer have conflicting information on minimum top offs and the light comes on. You would think you could fuel up however you wanted to.

 

[Mgrig]

I have almost 15K miles on my car and just got a check engine light. Took it to the dealer to be checked, bad O2 sensor. I thought it was kind of early to have a sensor go bad.

 

[mikec]

Many times the O2 error code really means something upstream is messing up the air:fuel ratio. An exhaust leak at the manifold can cause this - just loose fasteners for example. The O2 error code trips when the main computer basically can't get the O2 readings to where it wants them.

It can also be as simple as the connector to the O2 sensor wasn't seated fully and has vibrated loose.

As for filling the tank before much has drained causing "check engine" lights: one of the things OBD-II specifies is monitoring of the "evaporative emissions control system." This is what "captures" gas fumes from fuel evaporating in the tank and, in the old days, from the carburetor float bowls. It's basically a charcoal canister someplace - just like those "odor eaters" for shoes. To "monitor" the system, the way most cars do it is to verify a small vacuum builds up in the fuel tank & fuel lines/hoses as you drive - from the engine sucking gas out of the tank. When this vacuum gets large enough, a check-valve in the gas cap pops open, letting fresh air into the gas tank to keep it from collapsing in on itself. When the engine is warmed up and running, a "purge control valve" (don't know yet what Hyundai calls it; that's the generic name) connects the charcoal canister to the intake manifold so that the engine sucks out the trapped fuel vapors, cleaning the canister.

If something goes wrong in this system, it'll basically mean a hose broke/leaks, the canister leaks, or the purge valve doesn't seal properly... whatever, there will be an air leak to the outside that could allow gas fumes to leak out of the car. Vacuum won't build up in the system with such leaks; not seeing that vacuum is what trips the OBD-II check engine light.

If you do many short trips, if the gas cap doesn't seal well or wasn't tightened all the way, or if you re-fill often... the vacuum level never builds up enough to "satisfy" the OBD-II vacuum sensor and you get the check engine light.

mike c.