P0440 indicates the failure of the EVAP system to develop proper vacuum. If you got
and replaced a proper good quality gas cap, you have corrected the most common cause.
However, in fully searching the causes of this code (http://engine-codes.com/p0440_chevrolet.html ) (pardon the generic GM info, but will apply for you)-----you will find there are 5 other causes of the vacuum failure, which are independent of the gas cap. Your good local mechanic can sort those out; I would save your gas cap
receipt if you were advised to buy one not needed. As you probably know, your
State emission test facility checks your gas cap----so that is another way to find that specific portion of this answer,
(for their fee).
P0300 is a generic misfire code. For this, a specific Cadillac forum may be most
helpful = 2001 Deville intermittent service engine soon light code P0300
2001 Deville intermittent service engine soon light code...
I recently bought this car from an owner who took decent care of it but a week after i bought it, the service engine light appeared. After a few
View on www.cadillacforums...
Preview by Yahoo
You will see that this generic code is perhaps more serious, costly, and harder
to track down than the P0440. You will also discover that P0301 is cylinder 1 misfire, P0302 is cylinder 2, and etc, etc. If you get a good diagnostic, they will generally
be able to localize the problem fairly specifically---and get a more specific fix. The person who reported this code was unhappy because he was given several ($$) fixes,
before he got one that erased the code, and was a solution. (Be forewarned ----A generic misfire can have umpteen issues--or one! !; use your own local preferred method for solving this with a mechanic who gives you good service, and stands behind what they do.)
2001 Deville intermittent service engine soon light code P0300