What does it mean?
To extract the best performance from a given drop of fuel, the modern engine monitors and modulates a number of things, including oxygen content, spark timing, valve timing, injector pulse width, and many others. Among all of these, temperature is very important, both of the fuel itself and the intake and cylinders. Heat, of course, is generated by the combustion of the fuel, but it must be maintained within a certain range.
ECT Circuit Within the correct temperature range – depending on the vehicle, “operating temperature” is usually somewhere between 160 °F and 180 °F – fuel vaporizes efficiently, which enables complete combustion. Higher temperatures improve fuel vaporization, but only to a certain point. If temperature is too high, this can lead to engine ping, detonation, even damage.
On the other hand, if temperature is too low, fuel doesn’t vaporize or combust as efficiently, which means that the engine control module (ECM) has to inject more fuel than normal. Until the ECM determines that the engine has reached operating temperature, it runs the engine in “open loop,” that is, following pre-programmed parameters (fuel injection, spark advance, valve timing, etc.) for the given engine speed, load, and driver demand.
In open loop mode, the ECM does not use feedback from the AFR sensors to fine-tune performance and emissions. Once the engine reaches operating temperature, the ECM goes into “closed loop” mode, using the AFR sensors to maintain performance and fuel economy. Generally, the ECM expects that the engine will take a certain amount of time to reach proper operating temperature, but this can vary, depending on climate, coolant level, and thermostat function.
For example, if ambient temperature is 0 °F or below, the ECM may look for engine temperature to rise at least 70 °F within 15 minutes. At higher ambient temperatures, the engine should reach operating temperature, 160 °F to 180 °F, within 15 minutes. It seems counterintuitive, but the ECM doesn’t necessarily engine warming based entirely on ECT sensor readings, but also on the reaction of the air fuel ratio (AFR) sensors, as these are also active within a certain temperature range.
If the ECM detects, within a certain amount of time, that the engine has not reached proper operating temperature and can’t go into closed loop mode, it interprets this as a fault. The ECM will set a diagnostic trouble code (DTC) in memory and may or may not command the malfunction indicator lamp (MIL) to illuminate. DTC P0125 is defined as “Insufficient Coolant Temperature for Closed Loop Fuel Control.”