2001 volvo s440 1.9t

Volvo Radiator, Thermostat and Sensors Your cooling system's temperature controls include all coolant temperature sensors, Volvo thermostat, Volvo radiator or expansion tank cap, cooling fan(s) and fan clutch (if equipped). These cooling system parts function primarily independent of the engine but control the engine either through cooling or by sending control signals to your Volvo's electronic systems.
The Volvo thermostat is a spring-loaded valve that opens and closes based on the temperature of the coolant flowing through it. A high temperature reading followed by a drop to normal temperature (or a continuously low temperature) is a common first sign of a sticking Volvo thermostat. However, many other conditions may cause these symptoms, so you need to know how to eliminate each possibility.
The Volvo radiator or expansion tank cap is also a spring-loaded valve reacting to system pressure. It serves to maintain proper system coolant level at predetermined pressures. It must always be replaced with an exact replacement cap with the same pressure setting. Never use other caps except for short-term emergencies!
A belt-driven fan blade for pulling air through the Volvo radiator is usually on the Volvo water pump pulley and should have a fan clutch to control it. The fan clutch allows the fan to turn with the belt at low engine speed and "free-wheel" at higher speeds. A bad fan clutch either doesn't allow the fan to spin at low speed (overheating in traffic) or doesn't allow it to free-wheel at high speed (potential overheating on highway or reduced gas mileage).
An electric fan can be either by itself (usually front-wheel drive) or auxiliary (used with a mechanical fan). Both types are controlled via a temperature sensor - in the Volvo radiator or upper Volvo radiator hose or on the Volvo thermostat or Volvo water pump housing. This sensor is usually an on/off type switch with a fixed temperature setting. (Some vehicles may have 2-3 settings for multi-speed fans.) This sensor is commonly called an "auxilliary fan switch".
Other common temperature sensors are: 1) gauge sender (variable output); 2) warning light sender (on/off type); 3) lambda and/or fuel injection sensor(s) (variable to control fuel injection settings); 4) thermo-time switch (cold start valve control). Your Volvo may have other sensors as well.
Temperature control is critical to both performance and emission control. Unfortunately, this system is the most difficult to troubleshoot without proper equipment and diagrams. It's even more difficult with computers that adjust timing, idle speed, vacuum and fuel delivery automatically to make up for potentially faulty temperature sensor signals.
Maintenance of your cooling system sensors is virtually impossible since there's nothing really to "maintain". Keeping them clean both internally (coolant replacement) and externally (engine cleaning) is the best way to ensure trouble-free driving. Checking and replacing all parts at the factory-recommended time or mileage limits helps as well