MY 2004 VOLVO S40 COOLING FAN STAYS ON. AND NOW MY TEMPERATURE GAUGE IS ON RED BUT THE CAR IS NOT OVER HEATING AT ALL. CHECK ENGINE LIGHT IS NOT ON. PLEASE HELP

SOURCE: Volvo S40 OBD II P0171 & P0237

for p0171 it could be a few things like intake/exhaust leak,air system,map/vaf sensor,fuel presure/pump,injector(s),ho2s(heated oxygen sensor.

SOURCE: engine management light volvo s40 problem

The best thing is to go to your local Volvo maintenance shop or Volvo experts to find out whats the problem..usually the coomon ones I think is the programs inside its ECU unit..you try this..ask the volvo experts to reset the ECU and lets see..if problem arise then it is no doubt you might need the sensor..hope this help..

SOURCE: 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

SOURCE: 2004 Volvo S40 Noise and check engine light. Any

If the noise is a whistling or screeching type noise it means the breather assembly (integrated in the oil filter housing) has gone bad, its part of the PCV system. Good news is they are on an extended warranty now, I believe 10 years, 100K miles. Take it to the dealer for a free fix :)

SOURCE: How do you reset the check engine light on a volvo

The Check Engine light has nothing to do with changing the oil. The light is on because there is a problem with the emissions system. Even if you reset the light it will just come back on unless the problem causing it to come on is repaired. You will need to have the ECU scanned with an OBD2 scan tool. That tool will retrieve the code that has been set by the ECU. The code will tell you what part is being effected, but not necessarily what part or parts are defective. To determine that takes training, experience, and diagnostic skills. I suggest you seek professional help. If the engine is running fine there is no hurry but if it is not running as it should, then do not put this off.