I have 1993 Volvo 960 and the coolant senor is located on the back of the block ( next to fire wall) The access is very difficult because the are two water hoses and some electrical wires right next to it.
I posted pictures on the location and replacement of an ambient temp. sensor for a 2001 S40. Google "ambient temperature sensor location/replacement" or go to Matthew's Volvo site and search there.
Next to the thermostat. you will see it when you remove the hose
If you mean for Coolant level sensor it is on the bottom of the plastic coolant dish on the passenger side of the car.
I'm not sure what you're asking, but if you mean that you are getting an OBD-II diagnostic code of P0128 from your Volvo S40, that code corresponds to a problem with coolant temperature. On a first generation Volvo s40 (99-04), this usually means that the thermostat or electronic coolant temperature sensor are malfuctioning or have failed.
in mirror
On my 2000 International with a Detroit series 60 engine the water temperature sensor is on the back of the engine, passenger side in the cylinder head. Gears
On the passenger side of the engine compartment there is a surge tank remove that and the sensor is in the bottom. It will be the clear plastic tank where you add Coolant.
it's on the radiator, just pull the wires and unscrew it.
My 2002 has it in the drivers side mirror.
Usually right above your temperature control on your dash
driver side
I'm not 100% sure, but I would expect Volvo to keep them located in the same location and that would be in the Thermostat Housing. The part you would be looking for on your engine (where the thermostat is) would be screwed into it and have wires coming off it. This would be your Temp-sensor and is inexpensive to buy and easy to replace. Go to your local parts store to get.