Took my car to Autozone, because service engine soon light came on. Two codes came up p0445 and p0442 I changed the gas cap and now only p0442 is showing. It didn't pass emisions. What can I check?

Disconnect the negative battery cable for 30 minutes. Reconnect after and the code will clear. However if the problem persists, the code will come back with in 20 miles or 30 minutes.

http://www.obd-codes.com/p0442
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

code p 442 refers to evaporative emission (EVAP) system---small leak detected
it also refers to large leak detected --fuel cap off/loose === causes hose/connection,-- intake leak--EVAP canister---EVAP canister purge valve --mechanical fault --- EVAP pressure sensor

P0442 code means that the control module has detected a small leak in the Evaporative Emission (EVAP). The loose fuel tank filler cap is the most common cause that triggers the P0442 code.

Code P0445 indicates that there is improper purge flow in the EVAP system caused by a shorted purge valve.

Make sure Fuel tank cap is tightened well, then have it tested at Autozone, etc... If ok, then go back to inspection station.

P0445 - Evaporative Emission Control System Purge Control Valve Circuit Shorted is located in the engine compartment and is most likely the root of the problem.Would check the plug and wiring going to this to verify that no plug or wiring damaged has occurred.It is quite possible the part has gone bad.

there isnt usually a faulty gas cap code... there is evap system leak codes which could be caused by a faulty gas cap or filling up your gas tank while the truck running... if that was the problem and the problem is fixed then yes the codes will usually turn off after completing a few drive cycles

P0442 - Evaporative Emission Control System Leak Detected (small leak)

This indicates a fuel vapor leak in the EVAP control system. It means a very small leak has been detected. In fact, the leak can be from a hole as small as 0.04" in diameter. 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.

Symptoms: You likely won't notice any drivability problems.

Causes: A code P0442 most likely means one or more of the following has happened:
* A loose or improperly affixed gas cap
* A non-conforming gas cap (i.e. not factory/original brand)
* A small leak/hole in a fuel vapor hose/tube
* Other small leak in EVAP system
* Faulty vent o-ring seal

Possible Solutions: with a P0442, the most common repair is to:
* Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back.
* Otherwise, replace the gas cap, or
* Inspect the EVAP system for cuts/holes in tubes/hoses



P0445 - Evaporative Emission Control System Purge Control Valve Circuit Shorted.
Possible Solutions: with a P0445, the most common repair is to:
* Bad or loss contact wires
* Check harnesses, replace it


Hope helps (remember to rate this).

its an evap system small leak. most common cause is a loose gas cap. but could be any leak in the tank vent system

Sometimes you have to wait a week for the light to go off, the computer has to go through several start/run/stop cycles.

The computer that read the code for you can also reset it.  If you don't have one, you can go to your local auto parts store and have them reset it for you.  This is what your code means P0442 Evaporative Emission Control System Leak Detected (small leak).  This is what Causes it and possible fixes: CausesA code P0442 most likely means one or more of the following has happened:

• A loose or improperly affixed gas cap
• A non-conforming gas cap (i.e. not factory/original brand)
• A small leak/hole in a fuel vapor hose/tube
• Other small leak in EVAP system
• Faulty vent o-ring seal

Possible SolutionsWith a P0442, the most common repair is to:

• Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back.
• Otherwise, replace the gas cap, or
• Inspect the EVAP system for cuts/holes in tubes/hoses