Code P0440

[shazee]

Ok, so i coded P0440 for the second time  Usually i was able to fix it by just tightening or replacing the gas cap. I talked to a mechanic that coded it and he said that GM doesnt have any specific codes that determine whats malfunctioning. So im asking if anyone else has had this problem and knows what is most likely the problem. I know that the charcol box or a hose may not be working properly but if anyone knows what usually has the problem id love to find out before i take it in to get fixed. I have 194,000 miles on my 02 so if its just something clogged then i could see it after 8 years.

Thanks!

[MS03 2500]

That is a very generic code and it's not listed in the manual. It means you have a large leak in the system.
But here is what I was able to find out

Causes:

 Missing Fuel Cap
 Defective or damaged Fuel Cap
 Distorted or damaged Fuel Tank filler neck
 Torn or punctured Evaporative System hose(s)
 Defective Fuel Tank Sending Unit gasket or seal
 Split or damaged Carbon Canister
 Defective Evaporative Vent Valve and/or Evaporative Purge Valve
 Defective or damaged Fuel Tank
 Defective Fuel Tank Pressure Sensor

Description:

The Code P0440 indicates that there is a large leak in the Evaporative System, but this is somewhat misleading. What the code really indicates is that the Evaporative System will not create a significant vacuum when it performs its leak test. Let's go over a little about how the Evaporative Leak Test is performed by the Power Train Computer. When the leak test is performed, the vehicle must have been sitting for at least 4-8 hours in order for the engine temperature and air temperature to be identical. This is to provide a base line for the test because gasoline and diesel are volatile fluids that expand and vaporize easily with warm temperatures. When the Leak Test initiates, the Vapor Canister Vent Valve is closed to prevent any fresh air from entering the Evaporative System. Then the Purge Valve is opened which allows the engine to create a vacuum in the Evaporative System. After a specified time interval, which is usually about 10 seconds, the purge valve is shut off and the vacuum level in the system is measured with the Fuel Tank Pressure Sensor. Then a count down initiates which measures the rate at which the vacuum decays in the system. If the vacuum decays faster than the specified rate or, if only a small amount of vacuum is reached on 2 tests in a row, then the Power Train Computer will fail the Evaporative System for a large leak and set the code P0440. Now, what are some of conditions or components that will cause the Evaporative System to fail its leak tests?

[shazee]

ok, ill take it in to a shop. Depending on what it is, is this usually an expensive repair? I know that there are multiple conditions that makes the code.

[MS03 2500]

I really don't know repair cost, it has never happen to me. But I'd start by checking hoses if you were doing it yourself.

[anglarry04]

my 96 sonoma is doing that to me.   I was about to post it in the other vehicles section.  My 04 av had a similar code.   It was some other emission code.   very hard to diagnose.   In that case, I checked all visible vacumn lines.   I found nothing so I had the dealer troubleshot it for me.    I dont think ours is same issue though.  However, they charged me $40 (I think?) for a diagnosis fee.   As you said, have them troubleshoot, then repair yourself if you think you can do it and save some cash.   Thats probably gonna be my route for the sonoma