If the oil and coolant levels are okay and you aren't losing any fluids, then the most likely explanation is the thermostat. This appears to be a common fault on GM900s.
Check first that you haven't got an airlock and this in setting off the sensor. To check for air in the cooling system, run the engine in idle with the lid off the expansion tank and squeeze the top hose coming off the radiator. Watch out for your fingers on any moving parts. If there is air in the system it will bubble up into the expansion tank.
The other thing that could be setting off the Check engine light is a damaged vacuum pipe on the throttle housing. If this is split, you will be drawing air into the system there.
These two links should also prove helpful in diagnosing the problem:
motronic and
trionic which appears at Saabcentral.com
Check first that you haven't got an airlock and this in setting off the sensor. To check for air in the cooling system, run the engine in idle with the lid off the expansion tank and squeeze the top hose coming off the radiator. Watch out for your fingers on any moving parts. If there is air in the system it will bubble up into the expansion tank.
The other thing that could be setting off the Check engine light is a damaged vacuum pipe on the throttle housing. If this is split, you will be drawing air into the system there.
These two links should also prove helpful in diagnosing the problem:
motronic and
trionic which appears at Saabcentral.com