I have a Poulan PRO Model PB20H42YT. It was working great when slowly I lost forwar/reverse motion; freewheel control is engaged properly.

Engine kill when PTO engaged is an operator not in seat. Check/test the operation of the seat switch.

It sounds like there may be an issue with the drive belt on your Poulan Pro PO16542LT riding mower. When you engage the clutch, the belt tightens and provides power to the wheels, allowing the mower to move forward or backward. When you release the clutch, the belt should become loose and the mower should stop moving.
If the belt is loose or damaged, it may not be providing enough power to the wheels when the clutch is engaged. This could cause the mower to move slowly or not at all. When you release the clutch, the belt may not be loose enough to completely disengage from the wheels, which could cause the mower to continue moving slowly or even stall.
To check the drive belt, you'll need to consult the owner's manual for your specific model of mower. The manual should provide instructions for inspecting and replacing the belt if necessary. Alternatively, you may want to take the mower to a professional for repairs if you're not comfortable working on it yourself.

Maintanance - How to purge Hydraulic drive system Published 01/13/2010 11:56 AM ' Updated 06/02/2015 02:52 PM
How do I purge the hydraulic system?
Hydrostatic transmissions use oil, under pressure, to provide movement for the tractor. The fluid is pressurized with a pump inside the transmission. If any air is in the fluid, the pump will try to pressurize air. This condition is known as cavitation, and is detrimental to the pump since it allows the pump top operate dry. When the pump cavitates it is not pressurizing any fluid, so the transmission is not able to provide the movement needed.
If your mower is new, or has been stored for a long period (longer than 2 months) the transmission needs to be purged before use. Also, if you experience loss of drive, or slower response, then the transmission should be purged first.
The following procedure will remove trapped air inside the transmission(s).

IMPORTANT: SHOULD YOUR TRANSMISSION REQUIRE REMOVAL FOR SERVICE OR REPLACEMENT, IT SHOULD BE PURGED AFTER REINSTALLATION BEFORE OPERATING THE TRACTOR.
Step 1- Place the rear of the mower safely onto jackstands on a level surface with engine off and parking brake set.

Step 2- Disengage transmission by placing freewheel control, located at the back of the frame, in the freewheeling position.

2a- Mowers with seperate pumps and wheel motors the freewheel is activated by turnign the bypass valves, one on each pump, two full turns.
The oil resevoir needs to be filled to the specifications listed in the owners manual before starting the purge procedure.
Step 3- Sitting in the operators seat(riders), start engine. After the engine is running, move throttle control to slow position. With motion control levers in neutral (N) position, slowly disengage clutch/brake pedal.
Step 4-Move motion control levers to full forward position and hold for five (5) seconds. Move levers to full reverse position and hold for five (5) seconds. Repeat this procedure three (3) times.

NOTE: During this procedure there may be some movement of the drive wheels, but they are not under load. The air is being removed from hydraulic drive system.

Step 5-Move motion control levers to neutral (N) position. Shutoff engine and set parking brake. Check and add oil as needed for units with an oil resevoir.

Step 6-Engage transmission by placing freewheel control into driving position, or by closing the bypass valves on the pumps.
Step 7-Sitting in the operators seat, start engine. After the engine is running, move throttle control to full speed. With motion control levers in neutral (N) position, slowly disengage clutch/brake pedal.

Step 8-Slowly move motion control levers forward, after the tractor moves approximately five (5) feet, slowly move motion control levers to reverse position.
After the mower moves approximately five (5) feet return the motion control levers to the neutral (N) position. Repeat this procedure with the motion control levers three (3) times.

Step 9-Your mower is now purged and now ready for normal operation.For further information or assistance, we recommend contacting your lo

Check to be sure there is not a lot of grass and debris packed in spaces that prevent proper movement of the control arms.
If the control arms have free range of motion then you will need to check your fluid level and purge the pumps.
To purge the pumps jack the unit up and place on jack stands so that the drive wheels are off the ground. Release the transmissions so that they are in freewheel. Start the unit, release the brake and work the control levers slowly to full forward then full reverse 6 times. Shut the motor off, lock the transmissions back into drive. Start the unit and work the control levers back and forth 6 times again. Take down off the jacks and check fluid level. If there were any air pockets in the pumps this procedure would have purged the air out.

You can check the belt tention, between the deck and the chassis just to see if it is loose, it would reqire removing the deck to replace the belt.

could be the cover has slipped, if the outer casing moves it can reverse the polarity, see if it is loose, and if so move about whilst someone attempts to start

I hope you have fixed this by now. If you haven't It is possible that the ROS is malfunctioning. The tractor is equipped with a reverse operation system which allows the tractor to mow while backing up. It might be that the wiring in the switch or the shifter has become disconnected or broken.

For reference, the manual says this:

USING THE REVERSE OPERATION SYSTEM -

• Move motion control lever to neutral (N) position.

• With engine running, turn ignition key counterclockwise

to ROS "ON" position.

• Look down and behind before and while backing.

• Slowly move motion control lever to reverse (R) po si tion

to start movement.

• When use of the ROS is no longer needed, turn the
ignition key clockwise to engine "ON" position

Manual is available at: http://weborder.husqvarna.com/order_static/doc/NOUS/NOUS2007/NOUS2007-PB175H42LT-405327-96017000401.pdf

My immediate impression is that the float is "stuck open".

Pull the float-bowl, and check that:

a. the bowl and float are clean, and
b. the float moves freely, and
c. when it's in the "up" position, the flow of gas stops (which means that the needle is "seating" properly and doing its job)

Try loosening the belt tension to the transmission a little bit. Some of these are just stiff and you have to slip the clutch a little to rock them into gear.