What is the EVAP system and the FTP sensor . Service light is on and that was the diagnostic. EVAP canister or vent solenoid.

You have an Evap System problem and yes, this can cause poor gas mileage. Possible causes are:

• Plugged vapor canister
• Faulty vent solenoid
• Pinched or blocked EVAP hose
• Faulty FTP sensor

Testing the system an not replacing parts is the way to find the problem . Without knowing how ,using a factory diagnostic flow chart to go by would be best for a rookie .
System Description
The control module tests the evaporative emission (EVAP) system for a large leak. The control module monitors the fuel tank pressure (FTP) sensor signal to determine the EVAP system vacuum level. When the conditions for running are met, the control module commands the EVAP canister purge valve OPEN and the EVAP vent valve CLOSED . This allows engine vacuum to enter the EVAP system. At a calibrated time, or vacuum level, the control module commands the EVAP canister purge valve closed, sealing the system, and monitors the FTP sensor input in order to determine the EVAP system vacuum level. If the system is unable to achieve the calibrated vacuum level, or the vacuum level decreases too rapidly, this DTC sets.

Conditions For Setting the DTC
The EVAP system is not able to achieve or maintain vacuum during the diagnostic test.
Diagnostic Aids
• To help locate intermittent leaks, use the J 41413-200 Evaporative Emissions System Tester (EEST) to introduce smoke into the EVAP system. Move all EVAP components while observing smoke with the J 41413-SPT High Intensity White Light.
• A temporary blockage in the EVAP purge solenoid, purge pipe or EVAP canister could cause an intermittent condition. Inspect and repair any restriction in the EVAP system.
• To improve the visibility of the smoke exiting the EVAP system, observe the suspected leak area from different angles with theJ 41413-SPT .
• Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
• For intermittent conditions refer to Intermittent Conditions .

Inspect the EVAP system for the following conditions:
• A loose, missing, or damaged service port schrader valve
• A loose, incorrect, missing, or damaged fuel fill cap
• A damaged EVAP purge solenoid
Raise the vehicle on a hoist. Refer to Lifting and Jacking the Vehicle in General Information.
Inspect the EVAP system for the following conditions:
• Any disconnected, improperly routed, kinked, or damaged EVAP pipes and hoses
• A damaged EVAP vent solenoid or EVAP canister
Did you find and correct the condition?

Smoke testing the evap system to find the leak would be your best bet .

DTC P0455 Evaporative Emissions (EVAP) System Large Leak
The control module tests the evaporative emission (EVAP) system for a large leak. The control module monitors the fuel tank pressure (FTP) sensor signal to determine the EVAP system vacuum level. When the conditions for running are met, the control module commands the EVAP canister purge solenoid valve OPEN and the EVAP canister vent solenoid valve CLOSED . This allows engine vacuum to enter the EVAP system. At a calibrated time, or vacuum level, the control module commands the EVAP canister purge solenoid valve closed, sealing the system, and monitors the FTP sensor input in order to determine the EVAP system vacuum level. If the system is unable to achieve the calibrated vacuum level, or the vacuum level decreases too rapidly, this DTC sets.
There's more to it that just checking a hose ! Smoke testing the EVAP system is the best way to find leaks .

1. Inspect the Evaporative Emission (EVAP) System for the following conditions:
?€¢
A loose, missing, or damaged service port schrader valve



?€¢
A loose, incorrect, missing, or damaged fuel fill cap



?€¢
A damaged EVAP canister purge solenoid valve


2. Raise the vehicle on a hoist. Refer to Lifting and Jacking the Vehicle in General Information.
3. Inspect the EVAP system for the following conditions:

?€¢
Any disconnected, improperly routed, kinked, or damaged EVAP pipes and hoses

?€¢
A damaged EVAP canister vent solenoid valve or EVAP canister

Your best bet is take your vehicle to a ASE certified repair shop or the dealer , where they have the tools an knowledge .

DTC P0446: Evaporative Emissions (EVAP) Vent System Performance
Circuit/System Description
This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path. The control module commands the EVAP canister purge solenoid valve Open and the EVAP canister vent solenoid valve Closed. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum level has been reached, the control module commands the EVAP canister purge solenoid valve Closed and the EVAP canister vent solenoid valve Open. The control module monitors the fuel tank pressure (FTP) sensor for a decrease in vacuum. If the vacuum does not decrease to near 0 inch H2O in a calibrated time, this DTC sets.
Diagnostic Aids
?€¢
An intermittent condition could be caused by a damaged EVAP vent housing, a temporary blockage at the EVAP canister vent solenoid valve inlet, or a pinched vent hose. A blockage in the vent system will also cause a poor fuel fill problem.

?€¢
An EVAP canister, vent hose, or vent solenoid valve that has restricted flow may cause this DTC to set. Using a purge solenoid command with a scan tool will allow vacuum to be applied to the system instead of pressure. With the engine running, the EVAP canister vent solenoid valve open, and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 5 inches H2O.

?€¢
An EVAP canister filter that is restricted can cause this DTC to set. Refer to Evaporative Emission Canister Replacement .

?€¢
Disconnecting one component at a time while the EVAP system is under flow will help to pinpoint a restriction in the system

Do you have a scan tool that reads data PID'S ?
Your FTP - fuel tank pressure sensor could be at fault !

Ignition ON and fuel cap removed, verify that the FTP sensor parameter is between 1.3-1.7 volts. ?‡'
If not within the specified range, go to the FTP sensor section in Circuit/System Testing.

1. Install the fuel cap.
2. With the engine running, the EVAP canister vent solenoid valve open, and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 12 inches H2O.

?‡'
If the fuel tank vacuum is greater than the specified range, go to the restriction section in Circuit/System Testing.

If

Without testing we'd just be guessing . For each of these code's there is a diagnostic procedure for testing componets , electrical circuit's , smoke testing the system etc...
This DTC tests the evaporative emission (EVAP) system for a small leak. The control module monitors the fuel tank pressure (FTP) sensor signal to determine the vacuum decay rate. At an appropriate time, the control module turns the EVAP canister purge solenoid valve ON and the EVAP vent solenoid valve ON. This allows the engine to draw a vacuum on the EVAP system. At a calibrated time, or vacuum level, the control module turns the EVAP canister purge solenoid valve OFF, sealing the system, and monitors the FTP sensor input in order to determine the EVAP system vacuum decay. If the control module detects a leak larger than a calibrated amount, this DTC sets.
The following table illustrates the relationship between the ON and OFF states, and the Open or Closed states of the EVAP canister purge and vent solenoid valves.
the PCM detects an excessively rich condition, DTC P0172 sets.

I wouldn't think any of the thing's you mention as being the problem I would think the purge solenoid as being the bad But without testing that's a guess . Your best bet , take it to a qualified repair shop .

P0446 has nothing to do with the canistor !
DTC P0446 Evaporative Emission (EVAP) Vent System Performance
This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path. The control module commands the EVAP canister purge solenoid valve Open and the EVAP canister vent solenoid valve Closed. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum level has been reached, the control module commands the EVAP canister purge solenoid valve Closed and the EVAP canister vent solenoid valve Open. The control module monitors the fuel tank pressure (FTP) sensor for a decrease in vacuum. If the vacuum does not decrease to near 0 inches H2O in a calibrated time, this DTC sets.
DTC P0452 Fuel Tank Pressure (FTP) Sensor Circuit Low Voltage


How to troubleshoot an EVAP Pressure Sensor problem P0452 GM

EVAP Vent Solenoid Code P0446 The solenoid is stuck open !

Hello,The problem is that the EVAP system is not able to achieve or maintain vacuum during the diagnostic testThe control module tests the evaporative emission (EVAP) system for a large leak. The control module monitors the fuel tank pressure (FTP) sensor signal to determine the EVAP system vacuum level. When the conditions for running the test are met, the control module commands the EVAP canister purge solenoid valve OPEN and the EVAP canister vent solenoid valve CLOSED . This allows engine vacuum to enter the EVAP system. At a calibrated time, or vacuum level, the control module commands the EVAP canister purge solenoid valve closed, sealing the system, and monitors the FTP sensor input in order to determine the EVAP system vacuum level. If the system is unable to achieve the calibrated vacuum level, or the vacuum level decreases too rapidly, the PCM will set the code you now have.So, you either have a stuck open purge valve, a stuck open vent valve, a false reading fuel tank pressure sensor, or a cracked/broken EVAP line.

Continuous Memory Diagnostic Trouble Code (DTC) P0446 or P1450 indicates that Self-Test has detected the Evaporative Emission Control System has excessive fuel tank vacuum. Also, a DTC P0446 indicates a vent control malfunction.

Possible causes:

EVAP canister purge outlet tube (EVAP canister purge valve to EVAP canister) blockages or kinks.

EVAP canister tube (fuel tank to EVAP canister) blockages or kinks.

Fuel vapor elbow on EVAP canister contaminated.

Restricted EVAP canister.

Canister vent (CV) solenoid stuck closed (partially or fully).

Plugged or contaminated CV solenoid filter.

EVAP canister purge valve stuck open.

Fuel filler cap stuck closed (no vacuum relief).

VREF circuit open in fuel tank pressure (FTP) sensor.

Damaged FTP sensor. (fuel tank pressure)

Check for kinks or bends in the fuel vapor hoses/tubes (EVAP canister purge outlet tube and EVAP canister tube).

Visually inspect EVAP canister inlet port, CV solenoid filter and canister vent hose assembly for contamination or debris.

Check CV solenoid filter for blockage or contamination.

The common things I've seen are are mud in the canister and the canister vent solenoid being shot or the wiring to the canister vent solenoid being torn off. The resistance of the solenoid should be 48-65 ohms..
Pick up a Chiltons book and will give you a diagram of where exactly you find the canister. They are like $15 at any auto store. Good luck

Hi, the computer has sensed through the fuel tank pressure sensor that the vapor cannister is not evacuating properly. I have pasted details on the EVAP system below. Check the purge valve on top of the engine as shown below (#1 in picture). Make sure the electrical connector is on the valve, that the wires are not damaged, and check the hose for kinks. If all looks good, I recommend you replace the valve. A new valve is $23 at autozone. Please let me know if you have more questions and thanks for using FixYa.


Description & Operation
EVAP Canister
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.
EVAP Control System
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere.
The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, open, fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon.
The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
The EVAP system consists of the following components:
EVAP Purge Solenoid
The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. This normally closed solenoid is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The solenoid will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.
EVAP Service Port
The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid and the EVAP canister. The service port is identified by a green colored cap.
EVAP Vent Solenoid
The EVAP vent solenoid controls fresh airflow into the EVAP canister. The solenoid is normally open. The control module will command the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.
Fuel Tank Pressure Sensor
The FTP sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure = low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum = high voltage.

Continuous Memory Diagnostic Trouble Code (DTC) P0442 indicates that a fuel vapor leak has been detected [as small as 0.10 cm (0.04 inch)] in the Evaporative Emission (EVAP) system.
Note: Prior to performing the following diagnosis inspect the Canister Vent Solenoid (CV) for contamination. A situation may exist where contamination holds open the Canister Vent (CV) solenoid enough to set DTC P0442 and also plugs the port to atmosphere enough to keep system from being vented quickly.

• After market parts and accessories nonconforming to the EVAP system specifications.
• Damaged, cross threaded or loosely installed fuel filler cap.
• Small holes or cuts in fuel vapor hoses/tubes (EVAP canister tube, EVAP canister outlet tube or EVAP return tube) in the EVAP system.
• Loose fuel vapor hose/tube connections to EVAP system components.
• EVAP system component [EVAP canister purge valve, fuel tank pressure (FTP) sensor, canister vent (CV) solenoid, fuel vapor control valve tube assembly or fuel vapor vent valve assembly] seals leaking.
• CV solenoid partially open when commanded closed.
• EVAP canister leaking.
• Check for presence of fuel filler cap.
• Verify that the CV solenoid is properly seated on the EVAP canister (if possible).
• Visually inspect for cut or loose connections to fuel vapor hoses/tubes in following locations:
  • EVAP canister to EVAP canister purge valve.
  • EVAP canister to fuel vapor vent valve assembly.
  • fuel vapor control valve tube assembly to fuel tank (if applicable).