Seat Leon engine is hunting

Check the EGR is closing and check all the little pipes for security or leaks,
either vacuum leaks or excess exhaust gases present will give symptoms.

Could be air leak or the idle control valve. You can clean them. I use carb cleaner or if taken off tip some inside and give a good shake. Do this a couple of times. This can cause that problem.

Had the same issue, replaced the faulty oxygen 2 sensor (the one on the exhaust manifold), hunting still occurred, read online i needed to reset the ecu. I tried this exactly and het presto it worked, idle back to normal. now i dont know if doing ALL of the below helps but after doing it i was fixed

to reset your ECU, you have to do a little more than just disconnect the battery and plug it in again.

Disconnect the battery and leave disconnected for about 30 mins.

connect the battery back up and go put the key in the ignition.

Open all the doors, boot and bonnet and turn the key to get all the lights on. (This i am told alligns the throttle body)

Close all doors, bonnet and boot and get in the car.

Start the engine and turn everything electrical on, opening the windows full and then back up again. (This teaches the ECU what the maximum amount of electrical output maybe required from it.)

Turn everything off apart from the car and go for a spin