Cold start strategy relies on the Engine Coolant Temperature (ECT) Sensor operating properly to provide a function similar to a carburetor choke to richen the fuel mixture at cold start and during warmup.
The equivalent port fuel injection function at cold start is called wall wetting. Which means you have to inject additional fuel into the intake port at start because the injected fuel tends to coat the inside of the port, which is cold, instead of mixing with the air as the intake event occurs.
If the vehicle is equipped with gasoline direct injection (GDI) the fuel adder is very small compared to port injection because the fuel is injected directly into the cylinder. Better cold start control is one of the main reasons for DGI in addition to high pressure fuel atomization and mixing.
For port injection the cold start fuel adder, at freezing temperatures, can be huge in order to start the engine. So much so, that if the engine does not start you will eventually have to use the clear flood function in order to get the engine to start. Yes, if the engine can crank but not achieve engine position sync, or it drops out, fuel injection can flood the engine to the point of being too rich to start. As it turns out effective cold start strategies take a lot of dyno tuning (calibration), in a temperature controlled dyno cell to perfect, with some strategies being surprisingly complicate especially for 4-cylinder engines, which tend to run like crap during a cold start, as compared to an inline 6-cylinder engine for example.
A bad ECT sensor, as diagnosed by the ECM, should be recognized and set the MIL. However, if the sensor output is not open or shorted to ground, and the ECM input circuit is working, it is possible a valid voltage level is present, which would not set a DTC, and could negate the cold start fuel adder and your engine would not start when cold.
Generally, the ECT sensor is a highly reliable device, so if the ECM is not receiving a working signal then the input could be influenced by a short to another circuit or a very high impedance short to ground, any type of short which would put the ECT circuit within working range but still presenting the wrong temperature.
If you go Key On Engine Off first thing in the morning and use a PID capable scan tool for generic OBD-II, the ECT temperature should be very close to the Intake Air Temperature (IAT) sensor. In fact many times the same thermistor element is used for both, and even the input linearization circuit may be identical. The difference in the two being the ECT has a brass body (slow responding) and the IAT has a plastic body (fast responding, as compared to ECT). If the two temperatures are not within 5 degrees C then you have a problem with the ECT circuit, but no necessarily due to a bad sensor.
