Here are some possibilities:
* Faulty Thermostat (but likely more than just this): While a stuck-open thermostat is the most obvious culprit, a temperature this low points to additional problems. The thermostat might be open, but there's something else preventing the engine from heating up.
* Coolant Leak: A significant coolant leak would dramatically reduce the amount of coolant circulating, preventing the engine from reaching operating temperature. Check for leaks around the radiator, hoses, water pump, and engine block.
* Faulty Water Pump: The water pump circulates coolant through the engine. A failing water pump might not be circulating coolant efficiently, preventing heat transfer. Listen for unusual noises from the pump.
* Air in the Cooling System: Air pockets in the cooling system can impede coolant flow, preventing efficient heat transfer. This is often accompanied by overheating initially, but a severe airlock could drastically lower the temperature. Bleeding the cooling system might be necessary.
* Failing Radiator: A clogged or damaged radiator can hinder heat dissipation, but this usually leads to *overheating*, not underheating. However, a severely damaged radiator could prevent sufficient heat transfer causing this.
* Head Gasket Issues: While a blown head gasket usually causes overheating, a very minor leak could cause the coolant to bypass the engine, preventing proper heating. This is less likely given the extremely low temperature.
* Faulty Temperature Sensor: It's *possible* the temperature gauge or sensor is faulty, giving a false low reading. However, this would not explain the actual low engine temperature. If the gauge reads 100° but the engine feels extremely cold to the touch, the gauge is likely the issue.
What to do:
1. Check coolant level: Immediately check your coolant level in the overflow reservoir. Low coolant means a leak somewhere. Top it up (using the correct coolant mix for your vehicle) *only if* you can visually find no leaks. Driving with low coolant can cause catastrophic engine damage.
2. Check for leaks: Visually inspect all hoses, clamps, radiator, and the water pump for leaks.
3. Feel the radiator hoses: Are they cold even after running the engine for a while? Cold hoses suggest poor coolant circulation.
4. Listen for unusual noises: Listen for unusual noises coming from the water pump.
5. Professional Diagnosis: Given the severely low operating temperature, it is crucial to take your vehicle to a qualified mechanic for a proper diagnosis. This will prevent potential major engine damage. Trying to diagnose and fix this yourself without the proper knowledge could be harmful to both you and your car.
Ignoring this issue could lead to serious engine damage. Get it checked immediately.
