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Check engine light

I have a 2002 Nissan Frontier and the check engine light comes on when the gas tank is 1/2 empty till near empty but goes off when I fill up. Any ideas what to do!

Go into an auto parts store the next time it is on and have the codes scanned - many do this for free - big chains are most likely. Write down the exact codes (e.g. P0123) not what anyone says about them. Post the codes.

I will guess it is a weak fuel pump and or an old fuel filter that needs to be changed. Getting the code read and posting it back here will help us figure it out.

When you get the scan done, be sure to get the I/M Monitor icons that are flashing on the scan tool; as well as getting the trouble code(s)—like P0445.

Anyone know of site that has a list of OBD2 codes? I have a OBD1 list.

I have a 60 page list of ODB-II codes. Retrieved it years ago, so I cannot tell you where I found it.

BTW, what make and model is your ODB-I list for? The ODB-I system was not universal, and could be different for different models of the same make.

My money is on it being the EVAP system. My guess is that the vent line through which your tank breaths in as the gas level drops is having difficulty breathing in, possibly due to a saturated charcoal bed in the canister.

The next time the CEL light lights, remove the gas cap and reinstall it. If the light goes off my guess will be confirmed. But do not allow that to prevent you from getting the codes read and having a proper diagnosis done, just keep it as info.

Try this one oldschool:
http://www.aa1car.com/trouble-codes/

Thanks you guys, I just ordered a OBD2 text book from books4cars. $50.00. It sounds interesting.

My list is for the GM 95 S/T Truck,so you are saying it will not totally cover other OBD1 cars? When people post codes I am clueless because I don’t have a list.

I am in agreement with mountainbike that this problem most likely is due to a problem with the Evaporative Emissions System. However, only a scan of your OBD II system can confirm this.

If you are really lucky, your truck may just need to have an inexpensive valve in one of the EVAP system lines replaced. If you are not so lucky, it might be the carbon canister that is bad.

If it turns out that the carbon canister needs to be replaced, you should be aware that this is a pricey part. In order to avoid having to replace that part again, you have to remember to STOP filling the tank when the pump first clicks off. Those who insist on forcing additional gas into the tank after the pump clicks off are usually the ones who wind up paying $300.–$400. for a carbon canister replacement.