This is a frequent issue on older vehicles.

Here is the basic list of causes:

#1. Defective temperature sensor and/or gauge = verify, repair or replace.

#2. Low coolant level - Pressure test the cap and system, replace the cap and/or repair the system leak.

#3. Failed thermostat = replace with NEW.

#4. External radiator/condenser blockage = Condenser Coil Cleaner and water (external flush).

#5. Lime buildup inside the radiator = Citrus Flush the cooling system until it is clear.

#6. Failed engine fan clutch = replace with NEW.

#7. Failed Auxiliary electric engine cooling fan = replace with good used or NEW.

#8. Failed water pump = replace with NEW.

#9. Accessory drive failure = (Generic list) damaged/failed tensioner, idler pulleys, belt, Alternator, A/C compressor, power steering pump, emissions air pump.

#10. Harmonic balancer damage = repair or replace as needed.

#11. Failed cylinder head or gasket = Depending on many factors, replace with good used.

#12. cracked cylinder block = replace the engine with a good used.

#13. Transmission damage = repair or replace with a good used unit.

.

__________________
ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/