Hi all ~
I thought I would share this with you and hopefully it will help someone else with the same code.
I have a 99 Mustang V-6 (3.8L). The service engine light came on and I went to Pepboys to get the codes read. My car threw 2 codes, P0455 & P0411. After checking around and reading Mustang forums I found the problems.
The P0455 is a "Gross EVAP Leak" I found that the rubber gasket at the gas tank and filler tube was dry rotted and worthless. I replaced the gasket and all was good.
The P0411 code was a little more frustrating. I found that it could be several things. Make sure to check all vacuum hoses from the Secondary air pump and solenoid valve. Check the hoses for dry rot, disconnection or cuts. All my hoses were good. I then decided to replace the secondary air pump. This is located behind the plastic in the passenger front wheel well. I purchased an aftermarket brand for $104.00. The dealer wanted $260.00. I am taking my chances with aftermarket. After replacing the pump I disconnected the battery and reset the service engine light.
About 100 miles after replacing the air pump the service engine light came back on. I then decided to change the secondary air injection solenoid valve. This cost $50.00 from the dealer. The was an easy replacement. No tools required and it's located on top at the from of the engine. I decided not to reset the check engine
light, I would wait to see if it went off on its own. After about 100 miles the light went off.
My car is running great again. I hope this helps anyone out there with the same issues. Feel free to contact me if you need help.