I have a Husqvarna CTH 171. When i engage the blades they

MOVED YOU OUT OF CARS TO LAWN MOWER.
that is lost engine power
causes are
bad fuel or old and bad. fuel
all filters clogged,air or fuel
or carb clugged up,. (gunked up from bad fuel super common)

cold starts use a choke and goes super rich hiding clogged jets in carb.
clean carb or replace it if all above filters and fuel good.

Check all your pulleys for alignment and the blades for free turning. You may have a bad bearing.

depending on the age of the machine, there could be several possibilities
1-- belts are not tight enough--- belt tension system is not correct, wrong length belts
2--bearings for the shafts for the blades are rusted up and seized - preventing the blades from moving--- switch the machine off , disengage the belt drive and see if you can turn the blades by hand
3--- if you have an electric pto system to drive the belts to the blades then that system is not getting full power to engage the PTO or the clutch is not working--worn facings
take it to a service dealer and have a proper diagnosis done

YOu may have a damaged clutch mechanism or the drive belt might be slipping.
See this manual for part location.

http://www.husqvarna.com/ddoc/HUSI/HUSI2012_AAaa/HUSI2012_AAaa__96051002801.pdf

I would check the belt, the pulleys and the tension of the pulleys. Any of these could be the cause of your issue. Hope this helps!

Check the drive belt. This belt runs from the engine to the transaxle. Look for abnormal wear and if the sides of the belt look glazed/shiny. Check the pulley mechanism that tightens the belt for proper operation and belt tension while engaged.
Try reducing the speed selector. This is equivalent to running in a lower gear. Your tractor works hard just to turn the blade. Going uphill requires additional power, so some reduction in speed is to be expected.

Start by making sure the belts are actually on the pulleys and the tension pulley that engages when you go to engage the blades is functioning properly.

The mover has great vibration as all mowers do. That is why the valves are recommended to be regapped at every 250 hours of running time. If not checked the engine will run although will be reduced in power going up a grade. Eventually it will not start so easily if not serviced soon

Does engaging the pedal for the PTO lug the engine down, or does it just die like turning off the key?
If it lugs the engine down, then you need to make sure you can spin the spindle and idler pulleys by pulling the belt by hand. You should be able to slip it around the engine pulley enough with the blade pedal disengaged. Pull the belt up tight so the slack is around the engine pulley and try to spin the blade spindle and idler pulleys.
If it dies like turning off the key, then its likely a safety switch and may require a meter to test which.

The seat switch is has a shunt in the circuit that must be satisfied in order for the rest of the circuit to be happy. Make sure the connections are good, the pins inside the amp connectors will sometimes spread, push out of the lock etc. and if connection is lost will not work. Otherwise a new seat switch will be needed. You might check the PTO switch for continuity, without a diagram or schematic I am not sure how your circuit is wired. you have to have the parking brake engaged to get off the mower while the engine is running and the PTO is Dis-engaged?
I don't know if your "go" pedal eliminated this switch or not. But if with the parking brake engaged and the PTO disengaged, you lift off the seat and it dies, then its the seat switch.

Hope this helped Tim

A FEW THINGS TO LOOK AT ARE ,1THE PULLY SHIELDSMAKE SURE THEY AREN'T BENT TO CLOSE TO THE PULLEY'S.2 THE PULLEY BEARING'S MAY BE WORE OUT AND TO BREAK .3LOOK UNDER THE DECK TO MAKE SURE YOU DIDN'T GET SOMETHING TRAPED IN THE CUTTING DECK.HOPE THIS HELPED.