My car is now immoblized how can i reset it my car is a sabb 900 it is a 96 plate

Don't really know about the Sabb's. But if you don't have a rest in the fuse panel. Then turn the ignition on, press the fuel pedal three times within five seconds. The light should flash, when it stops, turn the ignition off. this should reset it.

was it tuned up too

my wild guess, is the alarm or immoblizer is active?
did you read the operators guide on starting?
so car only starts with the CEL off, will that is impossible
on most cars.
you key on, and the CEL must glow. if not.?
then ECU is out to lunch
or immoblizer is active.
i dont have the op guide, but they are free at all dealers, Acura.
and on line, if you google it.
why run a car with out the op guide. ?
what if you were stranded, and the answer was in the glove box.?
10x on newer cars. lots of complexity.

The valve regulating the water flow to the heat exchanger could be broken. It needs to be replace.

anit theft was it set off ? dealers can reset it ,or you could replace it ,my sabb went to dealer to reset codes.

dealer only encrypted software ,or it wouldnt be an immobilisor

heater coil in side dash has leak anti freeze is poison you could try stop leak in radiator with the heater on hot i like the powder type it might seal till you get the coil replaced

No.. However its a good idea to "reset" things by removing the negitive batter terminal for several minuets - then put it back on..

Have a great day

on the SID press customise then select system settings then service info and select reset service info. If the option to reset is not there you will need to change the date to a date beyond the 'Days to service', reset option will then appear. when reset, return date to correct setting. (to change date press and hold the info select button when the date is displayed.