I have a Schaerer Siena it is showing an error NTC Steam How do i fix

Sounds like the overheat stats on the boiler are burning out. Check the Klixons and replace, also the wiring connections to the boiler can sometimes get burnt out and require recrimping.

Replace fuse

There is a float switch in the water tank which is most likely sticking in the downward position. Take out the tank, at the back left you'll see a red piece of plastic. This is the float switch, jiggle it up and down and couple of times and ensure it moves freely, then fill the tank with water and check that the switch rises up with the water level. If this doent solve it, let us know.

Machines steam boiler below set point maybe hi limit tripped... bad relay.

Temp probe in the boiler replace

First You need to check the inlet water valve, I think the plunger is not working (sealing) properly and 2nd. Check the water level probe make sure is clean. If you don't feel comfortable doing this take it to a pro.

"level steam probe" occurs when it takes more than two minutes to fill the water in the steam boiler. The easiest solution is low water pressure to the machine. Does it have a water filter? replacing it will probably help. Other problems could be the fill valve is not opening fully and or scaled water lines going to the steam boiler. The first thing I would do is activate the hot water tap and look at the pressure. If the water is coming out slow then I know its a restriction to the machine.

replace the temp probe in the steam boiler

I dont know what model of coffee machine you have but I have a link for you Mr. Giovanni,kindly click the link below and choose what model you have then download the manual.
http://www.schaerer.com/download/default-n106-sE.html
If you find this helpful please dont hesitate to vote my answer.Or need any clarrification I can assist please comment this answer.Thanks and God bless.

With the machine off remove the grounds bin ( leave it out ) the press & hold the cleaning button until the de-scale symbol flashes