The ECU uses resistance as a measure of detecting broken sensors and faults. If you get an ABS fault it will be because it's detecting low signal (low resistance resistance). The problem with the ECU coolant temp sensor is the fact is works on resistance. If it completely and utterly breaks or it's unplugged it will throw up a fault. However, they tend to break down within certain resistance (or temperature) ranges and not show faults. They are extremely difficult to test, so just worth replacing.
The heat from the coolant changes the resistance in the sensor and relays it to the ECU which equates a fuelling level from the given resistance. Basically if it detects a low resistance (i.e detecting the engine is cold) it will up the fuelling mix to compensate. If it detects a high resistance (i.e engine upto temp) then it will lower the fuelling for maximum fuelling efficiency. Hence why the engine needs to be hot to control emissions when MOT testing
