my son has a 1997 nisson pathfinder. the gas pump didn’t shut off & it overflowed. then the check engine light came on. He went to autozone and code read was po440.I have read this on car talk but i can’t remember how to take care of this problem. can you please refresh my old memory. thanks for the help
P0440 OBD-II Trouble Code
Technical Description Evaporative Emission Control System Malfunction
What does that mean? This indicates that a part of the EVAP control system is no longer fuctioning correctly. The EVAP system consists of many parts, including (but not limited to) the gas cap, fuel lines, carbon canister, purge valve, and other hoses. The (EVAP) emission control system prevents the escape of fuel vapors from a vehicle’s fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine.
With a P0440 OBD-II trouble code, diagnosis can be tricky at times. Here are some things to try:
Remove and reinstall the gas cap, clear the code, and drive for a day and see if the codes come back.
Inspect the EVAP system for cuts/holes in tubes/hoses
Inspect for damaged or disconnected hoses around the Evap purge solenoid
Check and/or replace the sensor
Check and/or replace the purge valve
Have a professional use a smoke machine to detect leaks
Copied from obd-codes.com. Google is your friend. 
Or, in other words, you may need to replace the carbon canister. I hope that I am wrong, since this can be a very pricey part.
Start by removing the gas cap and retightening it. Give the OBD system a chance to reset itself (perhaps as many as 15 start cycles), and if you are lucky, perhaps the CEL will turn itself off. If not, then you may well be looking at replacement of the carbon canister (or if you are lucky, the purge valve!).
If it can be proven that you have NOT been topping off your tank(putting more gas into the car after the pump clicks off) and it was this ONE instance that damaged your evap system, you could bring it up to the gas station manager that they owe you a new part. Gas is high enough that we can’t have stations just throwing it away. You might want to contact your local EPA as well, if this has been happening a lot, then they’ll want to know about it