Emission control malfuction p0440 - 1999 Buick Century

Easier said then done without professional tools

However, I'll start you will some theory, then the shop manual procedures followed by some "common" problems

Theory:
Evaporative leak detection strategy is based on applying vacuum to EVAP system and monitoring vacuum decay. Powertrain Control Module (PCM) monitors vacuum level by means of fuel tank vacuum sensor input. At a predetermined time, EVAP purge solenoid and EVAP vent solenoid are turned on. This allows engine vacuum to draw a small vacuum on entire EVAP system. Whenever a sufficient vacuum level cannot be achieved, a large leak or faulty EVAP purge solenoid is indicated.

Conditions for setting this DTC are as follows:
No TP sensor, ODM, IAT sensor, or MAP sensor DTCs present.
Start-up ECT 40-86°F (4-30°C).
Start-up ECT not greater than 14°F (8°C) greater than start-up IAT.
Start-up IAT not greater than 4°F (2°C) of start-up ECT.
Fuel tank level is 15-85 percent.
BARO is greater than 75 kPa.
Vehicle has been driven at least 11 miles.
EVAP system is unable to achieve or maintain vacuum during diagnostic test.

Diagnostic Procedures
1.) Perform On-Board Diagnostic (OBD) system check.
After performing OBD system check, go to next step.

2.) Check if DTC P1665 or P1676 is also set. If any of DTCs is present, diagnose affected DTCs. If DTCs are not present, go to next step.

3.) Turn ignition off. Remove fuel filler cap. Turn ignition on. Using scan tool, observe fuel tank pressure. If fuel tank pressure is zero in. H2O, go to next step. If fuel tank pressure in not zero in. H2O, diagnose evaporative system.

4.) Replace fuel filler cap. Using scan tool, capture FAILURE RECORD data for DTC P0440 and clear DTC. Command EVAP vent solenoid on. Connect EVAP Pressure/Purge Diagnostic Station (J-41413) to EVAP service port. Pressurize EVAP system using diagnostic station. Monitor pressure on EVAP pressure gauge on diagnostic station. If pressure is 5 in. H2O, go to next step. If pressure is not 5 in. H2O, go to step 6.

5.) Maintain EVAP pressure at 5 in. H2O. Observe fuel tank pressure on scan tool. If fuel tank pressure reading on scan tool is 5 in. H2O, go to step 8. If reading is not 5 in. H2O, go to next step.

6.) Disconnect fuel tank vapor line and EVAP purge line from EVAP canister. Block canister fitting for fuel tank pressure line. Connect a hand-held vacuum pump to canister fitting for EVAP purge line. Ensure that EVAP vent solenoid is still commanded on. Apply vacuum to EVAP canister. If vacuum can be maintained at 5 in. Hg, go to step 11. If vacuum cannot be maintained at 5 in. Hg, go to step 10.

7.) Check for restriction in fuel tank vapor line or EVAP purge line. Repair as necessary. After repairs, go to step 16. If restriction is not found, diagnose EVAP system.

8.) Disconnect vacuum source line at EVAP purge solenoid and plug vacuum source fitting on solenoid. Using scan tool's output tests function, select and activate SYSTEM PERF. Pressurize EVAP system to 5 in H2O. Observe EVAP pressure gauge on diagnostic station while removing plug from EVAP purge solenoid vacuum source fitting. If EVAP pressure decreases to zero in. H2O within 15 seconds while system performance is activated, go to next step. If EVAP pressure does not decrease to zero in. H2O within 15 seconds, go to step 13.

9.) Install vacuum gauge on EVAP diagnostic station to vacuum source line. Start and operate engine to greater than 2000 RPM. Observe source vacuum level. If vacuum level is greater than -15 in. Hg, If vacuum level is not greater than -15 in. Hg, go to step 14.

10.) Check for disconnected or damaged vent hose. Check for damaged EVAP canister. Repair as necessary. After repairs, go to step 16. If no problems are found, go to next step.

11.) Check for faulty or missing fuel filler cap. Check for disconnected or leaking fuel tank vapor line. Check for disconnected or damaged EVAP purge line. If problem is found, repair as necessary. After repairs, go to step 16. If no problems are found, go to next step.

12.) Using scan tool, command EVAP vent solenoid on. Continuously attempt to pressurize the EVAP system by leaving the EVAP diagnostic station control knob in the pressurized position. Using the Ultrasonic Leak Detector (J-41416), locate and repair leak in EVAP system. After repairs, go to step 16.

13.) Replace EVAP purge solenoid. After replacing solenoid, go to step 16.

14.) Check for carbon release into the EVAP system. Repair as necessary. After repairs, go to step 16.

15.) Replace EVAP vent solenoid. After replacing solenoid, go to next step.

15.) Turn ignition on. Using scan tool, command EVAP vent solenoid on. Using EVAP diagnostic station, pressurize EVAP system to 15 in. H2O. Move rotary switch in diagnostic station to HOLD and observe EVAP pressure gauge. If EVAP pressure decreases to less than 10 in. Hg within 2 minutes, repeat step 3. If EVAP pressure does not decrease, system is okay.

Diagnostic Aids
Check for the following:
Cracked or punctured EVAP canister.
Damaged or disconnected source vacuum line, EVAP purge line, vent hose, for fuel tank vapor line.
Poor connection at PCM.
Inspect harness connectors for backed-out terminals, improper mating, broken lock, damaged terminals, or poor wire connection.
Check for intermittent in circuit.
Check for kinked, pinched or plugged vacuum lines to EVAP purge or fuel tank vapor line. Also check for restrictions.
Faulty or damaged evaporative canister.

P0440 OBD-II Trouble Code Technical Description Evaporative Emission Control System Malfunction

What does that mean? This indicates that a part of the EVAP control system is no longer fuctioning correctly. The EVAP system consists of many parts, including (but not limited to) the gas cap, fuel lines, carbon canister, purge valve, and other hoses. 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 P0440 could mean one or more of the following has happened:

• The gas cap is not installed or working properly
• The purge solenoid has failed
• The canister is plugged and not working properly

• Remove and reinstall the gas cap, clear the code, and drive for a day and see if the codes come back.
• Inspect the EVAP system for cuts/holes in tubes/hoses
• Inspect for damaged or disconnected hoses around the Evap purge solenoid
• Check and/or replace the sensor
• Check and/or replace the purge valve
• Have a professional use a smoke machine to detect leaks

That is a code for a loose gas cap or a bad seal on the gas cap.