My mother has a Mazda 6 and the check engine light has been on for some time. The most recent explanation the mechanic has come up with is that there is a problem with the fuel pump because my dad always tops the gas tank off instead of just stopping when the gas pump stops itself. Does this sound like BS to anyone else? How could this cause a problem?

 

What code is your car tripping? That should tell you right where the problem is.

Is the car driving fine, just the check engine light is on?

One time when I did top my Mazda3 off it had a rough start when I fired it up. (Although other times, no problem) I guess it's possible that you could fill it up to the point that you are restricting an internal vent line or something.

 

how much is the vehicle being topped off to? I normally can go about 25 cents over (when the pump automatically stops). I've never had any problems with a rough idle or a CEL coming on because of doing so. I've even had gas pour out a bit because of accidentally overfilling and still never had any problems.

I agree, you need to now what code is being triggered. Here is a list of the codes that are available for the mazda6 (Mazda 6 Trip Codes)

 

Are you turning the gas cap several clicks when you put it back on? A loose gas cap, or no gas cap, is capable of throwing a CEL.

 

Don't know the exact code, but I can find out. I think they had the loose gas cap problem on their protege, so they know that's not it. I'll check the code.

 

The code would be very helpful. It is possible to set a code by topping the tank to the top of the neck. You can set an evaporative code by doing this. When you top a tank off on about any newer car the evap system is very complex and can get raw fuel somewhere it is not supposed to be. Even if it is not an evap code, it could possibly set a rich code too.