code p0440 problem

My 2002 blazer check engine light came on, change gas cap reset computer light came on again 56 miles later. Dealer did "smoke test" found no leaks,dealer reset computer and said
drive it. LIGHT BACK ON Please help
Thanks, ZONE
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Info - Evaporative Emissions DTC P0440, P0441, P0442, P0446 #00-06-04-011A - (10/04/2001) Evaporative Emissions DTC P0440, P0441, P0442, P0446 1996-2002 Passenger Cars and Trucks
With Enhanced Evaporative Emissions Engines
Except 1998-1999 Chevrolet Prizm
This bulletin is being revised to add model years and diagnostic enhancements. Please discard Corporate Bulletin Number 00-06-04-011 (Section 6-Engine).
Enhanced Evaporative Emissions testing of 1996 and newer vehicles with enhanced evaporative emissions requires more stringent testing of the fuel system for evaporative emissions leaks. Vehicles with this system use an on-board diagnostic module (i.e. Powertrain Control Module (PCM) or Vehicle Control Module (VCM) as the controller. For the balance of this bulletin, we will refer to either of these components as a module.
Enhanced Evaporative Emission tests may be performed by the vehicle's module when specific conditions are met. These conditions are determined by the calibration within the module. These calibrations are vehicle platform and engine family specific.
When the key is turned to the start or run position, the module checks the output of various sensors on the vehicle to determine when and if the Evaporative Emissions tests should be run. Refer to Engine Controls sub-section of the appropriate Service Manual for specific DTC information.
Some of the most widely used parameters are listed below:
The Barometric Pressure (BARO) is greater than a specified kPa (psi). The system voltage is between 10 and 18 volts. The intake air temperature (IAT) is within specified temperature values. The engine coolant temperature (ECT) is within specified temperature values. The ECT and IAT are within specified temperatures of each other when the vehicle engine was started. The fuel level sensor indicates the fuel level is greater than a specified % of empty and less than a specified % of full. (For a 76 L (20 gal) fuel tank, a fuel level of more than 11 L (3 gal) but less than 64 L (17 gal) or as specified in the calibration). Some platforms such as the Alero and Grand Am, use a miles driven check as well. For the ECT and IAT to be within the specified temperatures of each other at the time of engine start-up, the engine will have to be in the non-running condition for a long enough period of time for the engine coolant temperature to drop from its normal operating temperature down to roughly 30C (90F). This could take several hours.
When the control module determines that the above conditions are met, the module tests for a leak in the EVAP system by:
Electrically closing the vent valve in order to seal the system. Electrically opening the purge valve which allows the running engine to draw a vacuum on the fuel tank. The module then closes the purge valve, resealing the system. The module then monitors the fuel tank pressure sensor and determines: If vacuum was achieved. If a leak is present, how large the leak is. If vacuum was not achieved. If one of the above tests fails, the module remembers that the system failed the test. If the module sees a failure of the EVAP test for two consecutive tests, the module will set a trouble code for EVAP leak (DTCs P0440, P0442, or P0446).
Important If the PCM is completing the EVAP diagnostic, filling of the fuel tank cannot be completed as the vent valve will be closed and the fuel dispenser nozzle will sense the pressure build-up and shut down. This is the expected result. No parts should be replaced as nothing is faulty. To keep from encountering this situation, it is imperative all customers and dealership personnel be advised NOT TO FILL THE FUEL TANK WITH THE ENGINE RUNNING. If the condition is encountered, turn the ignition to the OFF position for a minute. This will allow the PCM to go to sleep completely, thereby opening the vent valve.
zone wrote:

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