Car problem

[doodle]

It is saying to you "sell me and buy a bicycle" :-)

[MediumTex]

Check to make sure the gasoline fill cap is sealing tight. If air is getting into the tank and remaining there, it causes the check engine light to go on. Presumably, when the tank is full, there is no air, and the light goes off. Otherwise, air is probably leaking in there somehow.

You might just buy a new fuel cap from the dealership and see if that helps.

Also, you might run a few tanks of super unleaded or regular unleaded with a fuel additive.

I've had pretty good luck in high mileage vehicles in keeping the check engine light off by using super unleaded.  Apparently, the typical trigger for the light is a sensor in the emissions system getting dirty, and the slightly cleaner burn seems to help.

Your problem sounds more like it is a loss of pressure in the fuel system, and the most likely culprit for this seems to be a defective gas cap.

Make sure you get one from the dealership if you go this route.  You want to make sure that you are in-spec on this item since you are trying to eliminate the fuel cap as the culprit.

If you get a new fuel cap and the check engine light still comes on, you might try disconnecting the battery for about 30 minutes to let all of the codes clear out and then reconnect it.  This seems to help with some of these check engine light situations.

As a final option, you might just make sure that you never get below half a tank of gas. 

[hoost]

The gas cap seems like a good initial thought.  I would say the best thing to do is to swing over to an autozone or similar auto parts store and have them pull the fault codes next time the light comes on.  Having those codes will help to narrow down the problem a lot more easily.  They will normally do it for free and give you a print out of the codes.  Emissions could be an issue, but it usually takes a lot longer for the code to reset because there is a minimum cycle of normal operation required before it will clear (at least on the cars I've had).  The other thing that comes to mind could be related to fuel pressure, which may be related to the fuel pump going out or the fuel filter getting dirty.  As the amount of fuel in the tank decreases, there is less head pressure above the pump which would reduce the output pressure of the pump.  Or, it could be something completely unrelated.  It's hard to say without knowing the code, and there's no use in beginning to change out parts without knowing what fault the engine is giving.

[MediumTex]

I will try the gas cap, and will get the code pulled again next time it comes on.

Try a few tanks of super unleaded too.

[KevinW]

FYI, OBD2 computers such as yours store the last few trouble codes (something like 5). So you should be able to get the code read even if the light is not currently on. Provided you haven't rebooted the computer by disconnecting the battery.

If you want, you can buy your own scan tool for about $60:
http://www.amazon.com/Actron-CP9550-OBD ... 737&sr=1-6

I'm scratching my head on why any of the Causes listed here would be related to fuel level in a vehicle with an electric fuel pump:
http://www.obd-codes.com/p0420

[Bob]

I had a similar problem on my 2006 Jeep Liberty.  I kept getting a "gas cap" error message with an annoying "ding" warning sound about every 10 minutes.  I replaced the gas cap and that didn't work.  Then, I talked to the dealer and learned how to reboot the computer:  disconnect the cable from the positive battery terminal and hold it for 15-20 seconds on the negative battery terminal.  That worked like a charm. 

[AgAuMoney]

Lat 1990's ford ranger.

Several codes that seemed to be related to fillups -- secondary emission system, lean burn, O2 not responding / too hot / lean, and it seems like there might have been another.

Fuel cap issue.  I cleaned (using wd-40) the mating edges of the tank and fuel filler opening.  The opening had some hard dirt buildup caused by or causing air leakage.  On the clean surfaces of both pieces I applied a generous film of Vaseline.  That stopped the "secondary emission control system" codes.

Still had others that kept coming up.  Did a reset with the code reader.  They'd come up again a few days later.  Found out thru some online resources and verified with a mechanic that since Ford used engine vacuum to suck fumes out of the fuel tank, if the cap leaks (or more commonly a vacuum hose), it can cause a lean burn on that side of the engine, and that will cause a code.  It can also cause an O2 sensor code on that side of the exhaust, and perhaps even burn out that O2 sensor.  That would be the other code.

The mechanic suggested I reset the codes for another tank of gas, and if it kept coming back, disconnect the battery for a few hours and run another tank of gas thru it.  Then if it still kept coming back, it would be time to replace the O2 sensor.  I didn't have to disconnect the battery or replace any parts.  I haven't seen a code since July and passed the emissions test in August.