If only engines weren't reliant on temperature, there'd be no need for a coolant temperature sensor. But the presence of a sensor does offer benefits that help car owners feel relaxed when driving. In the old days, drivers only had an automatic carburetor choke to adjust fuel mixture depending on the engine's temperature. Now, it's the engine's powertrain control module (PCM) that does all the work. Long gone are the days when carburetor chokes had to be adjusted manually for optimum engine performance. Today's engines have electronic fuel injection systems and ignition systems without distributors. It's the PCM's job to control these systems to ensure smooth drivability. Still, the module needs the coolant temperature sensor to determine when it has to adjust fuel mixture and advance or retard spark timing. Adjusting the spark timing keeps your car's engine running smoothly. During cold starts, the PCM enriches fuel mixture and delays spark timing to make the engine drivable. Once it reaches normal operating temperature, the PCM leans out the fuel mixture and advances spark timing for normal operation. So you see, if your car's coolant temperature sensor breaks down, there's going to be a lot of problems: cold drivability issues, irregular idling, poor fuel economy and lack of power. Fortunately, all you have to do is hook up a scan tool to your onboard diagnostics port to figure out what's wrong. If the coolant sensor has failed, just get online and visit Car Stuff. We've got temperature sensors readily available.