When l drive around town or when using more gas the engine temp gauge is normal but when cruising on highways when the outside air temperature is about 35 degrees F or below the engine temp gauge falls. Sometimes it goes all the way down. In summer driving the temp gauge is always at normal. The vehicle and engine is a 1999 Saab 9-5 wagon with 2.3t four cylinder. Please help. Thank you.
When l drive around town or when using more gas the engine temp gauge is normal but when cruising on highways when the outside air temperature is about 35 degrees F or below the engine temp gauge falls. Sometimes it goes all the way down. In summer driving the temp gauge is always at normal. The vehicle and engine is a 1999 Saab 9-5 wagon with 2.3t four cylinder. Please help. Thank you.

***l forgot to mention that it takes longer than most cars for the temp gauge to reach normal operating temp. Also, is it bad to operate the car if the temp sensor or thermostat are not working properly?

Your coolant temperature sensor is on the fritz or your thermostat is stuck partially open. Both are relatively cheap to replace, so I'd just recommend having both replaced, especially if they're 10 years old.

Thermostat is probably stuck open. Not a very good situation for the car. It needs to get warm otherwise there will be fuel mixture and/or emission issues & trouble. Replace it with 82' C thermostat made by Wahler. It should then warm up & stay in the normal zone. This would be a good time to also flush & replace the coolant. Only use Saab coolant as it needs to be the approved type, Otherwise there may be head gasket problem in near future. Do not mix coolant of different types. Read & follow owner is manual.

Your thermostat it bad try replacing that and change your coolant.

try putting a cardboard piece the cover part of the Radiator this my help do not cover it all. l do this on my 69 charger when it cold and im driving it in the winter