IIRC there is a sensor that senses pressure build up in the tank and when it gets to a certain point it releases some of the pressure by letting some of the fumes out into the engine. You dont have to worry about it causing the car to run differently but possibly see if the cap is cracked or sometimes if you dont push the cap down all the way and when you twist it will feel as if its on but not seal quite right. Im a lil stumped.
since thats all ive had to ever do on my 2000 grand am, the cap on that is very hard to push down and i never quite get it tight all the time.
P0440 is a large leak in the EVAP system..alot of people call this the "gas cap code"
A O2 sensor will not fix the problem.
What has happend is the PCM (computer) has seen a leak in the EVAP sytem and has turned on the light.
Here is how the PCM runs the test
The evaporative emission (EVAP) system is checked by applying vacuum to the EVAP system and by monitoring for a vacuum decay. The Powertrain Control Module
(PCM) monitors the vacuum level through the fuel tank pressure sensor
signal. At an appropriate time, the EVAP canister purge valve and the EVAP vent solenoid are turned ON, allowing the engine to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP canister purge valve will turn OFF, sealing the system. DTC P0440 will set because the PCM is not detecting the expected vacuum. This can be caused by the following components with the conditions listed below:
The fuel cap for the following conditions:
The fuel tank pressure sensor for the following conditions:
The EVAP purge line for the following conditions:
The EVAP vent hose for the following conditions:
The fuel tank vapor line for the following conditions:
The EVAP canister purge solenoid valve for the following conditions:
The EVAP vent solenoid for the following conditions:
An open ignition feed circuit to the following components:
EVAP vent valve
A damaged EVAP canister
A leaking fuel sender assembly O-ring
A leak in the following items:
Fuel filler neck
If you still are having problems i would recommend taking your car to a shop and have them diagnos the problem, the dealer has the tools necessary to diagnos the EVAP system properly and repair your problem.