IF you just filled the fuel tank. Check your gas cap.
Info from GM eSI.
Adm Msg -- P0440 Caused By Gas Cap Not Being Tightened After Fuel Fill
2002-2003 Chevrolet TrailBlazer
Equipped with LL8 4.2L Engine or LM7 5.3L Engine
Service Manager
Some customers may comment about a Check Engine light with a P0440 series DTC indicating that an evaporative emission leak has been detected. After reviewing the warranty data and making dealer callbacks, General Motors Service Operations has found that a very high percentage of these DTCs are caused by the gas cap not being completely tightened after a fuel fill. There is no known issue with gas cap retention. Therefore, if a dealer should get a vehicle in with this failure, and after running through the SI diagnosis no other trouble is found, please ensure that the gas cap is tight. Instruct the customer that this issue may have been caused by the cap not being tightened after the last fill up. In addition, to ensure that customers do not experience a similar incident with the first 200 miles of ownership, please ensure that the technicians performing the PDI or initial tank fill up at your dealership are tightening the gas cap completely. To make sure the gas cap is secured completely, you must hear at least 3 clicks.
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.
