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Discussion Starter #1
So yesterday my CEL came on. I thought gas cap. I took it off and put it back on nice and tight, 4 times over the course of about 1 hour. The CEL stayed lit. Then this AM I fueled up, made sure the cap was on tight. Light stayed on.

I went straight to the dealer, who very kindly accomodated me immediately since I have an HPDE coming up.

they called me a while later to say that the code was for fuel system evaporative thingummy, usually caused by a loose gas cap, that they've cleared the code, tightened the cap and presented me with a bill for 30 minutes labor.

I was happy to pay it since they did accomodate me out of turn and I'll be able to go to HPDE, but I am confused.

1. Why didn't the CEL go out after I had attended to the cap?

2. If the CEL doesn't go out immediately but over time, does Lotus expect us to keep driving the car till it goes out and not head for the dealer?

3. Or is it that the light won't go out till the dealer clears the code? if so, that would mean I have to go to the dealer every time the cap works itself loose and shell out for labor?
 

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I have looked into this in the past, when I have had similar issues with some of my other cars. (I haven't had the problem with my Lotus, but I have had bad gas caps on a couple of my other cars.) My understanding is that OBD II shows a fault after detecting the same issue on two separate occasions. The check typically takes place only once a day, however. Meaning you can make several trips in a day, and it only conducts one check. This is why you can fill up with gas, and then, a couple of days later, the CEL turns on. It doesn't turn on right after you fill up because it takes a couple of days.

On the other hand, the CEL will only turn off once OBD II has detected that the fault is gone three times. This will usually take several days. That is why you have to tighten the gas cap, then wait a few days. This is also why service departments and owners manuals usually tell you to give it a few days after attempting to fix the problem before bringing the car in to the shop to have it checked.
 

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Discussion Starter #3
Thank you for your reply.

My Honda will say "check Fuel Cap".

on the Lotus, a CEL may not be the gas cap. Does that mean we keep driving for 3 days with the CEL and no harm will come to the car in case its something other than the gas cap?

I appreciate the input!
 

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Buy a OBDII reader so you know what the codes are and save $$ by not visiting the dealer...
 

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Discussion Starter #5
Well, doh. Now I feel like a real ass.

considering I used to pull my own codes all the time on other (pre OBDII) cars in days gone by. :wallbang:
 

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It is true that a loose fuel cap can cause the CEL to illuminate. If that indeed was the cause, simply tightening the cap will not turn off the light right away. As OBDII is designed, the ECU runs it's own self tests called monitors. One of those is the evaporative emissions system monitor. That self test only runs under certain conditions,eg., more than 15% fuel level,but less than 85%. There are also ambient temperature criteria,(Don't recall the exact,but for the sake of argument greater than 40 deg and less than 100),that must be met. Once the requirements are met and the ECU runs it's monitor and passes, the ECU would then turn out the light. This is slightly oversimplified,but that's basically how it works.
 

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I think the reason it did not turn off is because your not getting the true lose gas cap error. That error is called "gross leak detected". That CEL will turn off a few days after you "fix the leak"
 
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