My Pontiac G6 Sunroof is stuck in the vent position.

First off this is controlled by a module (computer) The switch would only have five volts . 5volt reference voltage from the control module . Not knowing how to test the electrical circuit's for this could be expensive . Short out the module or 5 volt ref. voltage circuit inside the module.
Your best bet , take it to a qualified repair shop . Look at a wiring diagram here http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model and engine size . Under system click on body & accessories ,then under subsystem click sun roof . Then click search button then the blue link . Now tell me you under stand what's happening here .

Power Sunroof System Components
The power sunroof system consists of the following components:
• Sunroof control module
• Sunroof Opening Position Switch (SOPS)
• Sunroof switch
• Sunroof motor/actuator
• Sunroof glass (spoiler type, with glass sliding above roof)
• Sunshade
• Rail and track assembly upon which the sunroof glass rides
• Power supply for the sunroof control module: SUNROOF 20 A fuse
• Ground from G302

Sunroof Opening Position Switch (SOPS)
The SOPS tracks the position of the glass through 2 limit switches:
• Soft stop limit
• Vent limit
The sunroof control module monitors the limit switches and based on the inputs determines the position of the sunroof glass. As a limit switch closes, the 5 volt reference will be shorted to the sunroof limit switch low reference circuit, pulling down the 5 volt reference to 0 volts.
The sunroof control module understands the position of the sunroof glass based on the different input combinations through the sunroof limit switch soft stop signal and sunroof limit switch open vent signal circuits.

Power Sunroof Does Not Close
Perform the Initialization/Teach Process any time a new sunroof motor actuator is installed in the vehicle or any time an existing sunroof motor actuator is removed from the vehicle. Refer to Sunroof Motor/Actuator Initialization/Teach Process .

Do you know how to do automotive electrical testing ?????? How to read a wiring diagrams ???? http://www.bbbind.com/free_tsb.html .Videos on youtube Ohms Law Tech Tip from L1 with Dave Hobbs Copy of Voltage Drop Essentials TST Motor Age The only way to find the problem , testing !

Sunroof Description and Operation (One Connector System)
Sunroof
This diagnostic procedure is to be used for the 1 connector Sunroof System that combines the sunroof control module with the sunroof motor/actuator.
The Sunroof System consists of the following components:
• BATT MAIN 2 Fuse 50 A
• Body control module (BCM)
• RAP relay
• S/Roof Fuse 20 A
• Sunroof module (includes sunroof motor/actuator)
• Sunroof switch
The Sunroof System is controlled by the sunroof switch and the sunroof motor/actuator assembly.
When the ignition is turned to RUN or ACCESSORY, or when the retained accessory power (RAP) system is active, the body control module (BCM) applies a ground to the RAP relay in the instrument panel (I/P) fuse block. This activates the RAP relay and allows battery positive voltage to be applied through the SUNROOF Fuse to the sunroof module. Ground for the sunroof module is supplied from G302.
The sunroof can be placed in 3 preset positions:
• Open
• Vent
• Closed
When the sunroof switch is placed in the OPEN position, a ground is applied from the sunroof switch low reference circuit , through the sunroof switch and to the sunroof switch open signal circuit. The sunroof motor/actuator then applies voltage to the sunroof motor open control circuit and applies a ground to the sunroof motor close circuit. The sunroof motor then opens the sunroof to the full open position.
When the sunroof switch is placed in the VENT position, a ground is applied from the sunroof switch low reference circuit , through the sunroof switch and to the sunroof switch open signal circuit. The sunroof module then applies voltage to the sunroof motor open control circuit and applies a ground to the sunroof motor close circuit. The sunroof motor then opens the sunroof to the VENT position.
When the sunroof switch is placed and held in the CLOSED position, a ground is applied from the sunroof switch low reference circuit , through the sunroof switch and to the sunroof switch close signal circuit. The sunroof module then applies voltage to the sunroof motor close control circuit and applies a ground to the sunroof motor open circuit. The sunroof motor then closes the sunroof.
The sunroof can be placed in any preferred open position when:
• The sunroof is in the fully opened position by pressing and holding the sunroof switch in the CLOSE position until the panel has reached the desired position then release the CLOSE switch in order to stop the sunroof
• The sunroof is opening, it can be stopped by momentarily pressing either side of the switch.
The sunroof control module, sun roof actuator, and the sunroof motor are integral parts of the sunroof module assembly and are not service separately.

Your best bet , take it to a qualified repair shop or buick dealer .

Just guessing here. Maybe it has a safety feature like an elevator door. If something is causing resistance while its trying to close it stops and opens back up. Try inspecting the tracks the sunroof runs on for anything stuck in them, or and damage.

it sound like there is a problem with the cables _ they either streched or catching on something or just could be worn out also

Do you know how to test automotive electrical circuit's with a DVOM - digital volt ohm meter ? When adjusting the moon roof did you see any switches ? sunroof limit switch
Measure the reference voltage of the sunroof limit switch soft stop signal circuit at the sunroof control harness connector to a good ground.
Does the reference voltage measure near the specified value?
4.5-5.5 V
Notice: Do not open the sunroof glass past the full open soft stop position. Doing so may damage the sunroof assembly and or the roof top.
Connect a jumper wire to the sunroof limit switch soft stop signal circuit at the sunroof control module harness to a good ground.
Turn ON the ignition, with the engine OFF.
Using the sunroof switch, operate the sunroof system, not to exceed the full open soft stop position.
Does the system operate as requested?

Do you know what a wiring diagram is ? Free one's here
http://www.bbbind.com/free_tsb.html Enter vehicle info. year ,make ,model an engine size . Under system click body & accessories ,then under subsystem click on sun roof . Click the search button then the blue link . See the control module , motor an the two switches ? One is probably miss adjusted . or bad !

Power Sunroof System Components
The power sunroof system consists of the following components:
• Sunroof control module
• Sunroof motor with the limit switches
• Sunroof switch
• Sunroof glass-spoiler type, with glass sliding above roof
• Sunshade
• Rail and track assembly upon which the sunroof glass rides
• Power sunroof system circuit breaker: PWR WDO 25 A
• Ground: G204

Sunroof Limit Switches
The sunroof limit switches track the position of the sunroof glass through two limit switches:
• Soft Stop limit switch through the sunroof limit switch soft stop signal circuit
• Vent Stop limit switch through the sunroof limit switch range signal circuit
The sunroof control module monitors the limit switches and based on the input combinations, determines the position of the sunroof glass. As a sunroof limit switch closes, the 5 volt reference will be shorted to the sunroof limit switch low reference circuit, pulling the 5 volt reference to 0 volts.
The sunroof control module monitors both sunroof switch input requests and based on the sunroof limit switch input combinations, will control an output to the sunroof motor, to drive the sunroof glass to the requested position.

Sunroof Motor
The sunroof has a bi-directional motor. When the motor is not operating both circuits are grounded. When the motor drives the sunroof glass in the direction from the closed to vent or vent to open position, the following occurs:
• Voltage is supplied to the sunroof motor open circuit.
• Ground is maintained to the sunroof motor close circuit.
When the motor drives the sunroof in the direction from the open to vent or vent to closed position, the following occurs:
• Voltage is supplied to the sunroof motor close circuit.
• Ground is maintained to the sunroof motor open circuit.
The sunroof control module is equipped with a timer circuit which stops the operation of the sunroof motor after 8 seconds.

by using a simple test light you can check each post individuly with the switch in each position also at the motor end check where the wires go into the motor have someone work the switch in each position and test each wire if you have fire going to the motor corrsponding to the switch position (you should have fire on at least one wire in both closed and another wire in the open position of the switch at the motor) and if so then your motor is probably out

Lubricate

clean out the channels!

The sunroof continues to open and close normally from front to back, but the vent stays open.

It sounds like it just needs to be reinitialized. Turn your key to the on position with motor off and hold the up/full vent button down for at least 30 seconds or until your sunroof either clicks or goes to the full vent position. That should fix your problem.