BMW 3 Series engine temp

yes if the heater matrix is leaking there is gonna be no coolant left in the system eventually so the coolant sensor won`t get a reading,you need to rectify the coolant leak before you cause damage to the engine,keep topping up the cooling system untill you can sort the leak

Check if it's rain water or coolant inside car . Rainwater clean , coolant coloured .Test and replace thermostat if faulty .Are you losing any water from coolant header tank ?

hi yes its loosing water; i have changed thermostat and housing, and water pump.it does about 50-60 miles b4 the water light comes on. ive been told its alot of work to get to the matrix heater, and if i disconnect it then the heaters wont work anyway will they? and would this cause the engine temp to drop? thanku

If it's the heater matrix that is leaking you could bypass it , but wont have any heat . It will stop water / coolant leak if that's where it's coming from . On some models it was not bad to replace i.e they can be changed without dash removal. You had to take out glove box and I think it then slides sideways, but not 100% .Might be worthwhile making sure head gasket is o.k by having a block test ( sniff test ) as by running with coolant loss , can do in gasket .