Q: Can a faulty sensor keep the engine from starting?

Today’s automobiles use many different sensors to signal engine controls and computer circuits to start a vehicle’s engine and run all the programing for fuel, timing, and emissions control. Bearing this in mind, it is critical for the vehicle’s main computer the PCM (powertrain control module) to receive the proper signal to carry out the task. One factor that is very important to remember is that the computer reacts to electrical inputs that it receives and responds with an output or action that its programed to perform. Keeping this in mind, you should realize that the computer makes calculated decisions but does not second guess the information provided. It is able to tell if values are out of the guide parameters, usually setting a diagnostic trouble code and perhaps the Check Engine Light.

An example of how the information is seen and processed would be a coolant temperature sensor sending a voltage value to the PCM, indicating that the engine is at a certain temperature. Since the signal is a voltage value that can be affected by either a faulty sensor, its connection to the wiring, or the wiring itself, it is possible for the computer to receive a signal for an improper value. Since the wiring harness is subjected to temperature and movement as the vehicle is driven, these factors can affect the outcome of the signal received.

If a voltage signal is sent indicating a reading equal to a cold engine of 20 degrees, even though the actual temperature of the engine may be well over 100 degrees, the PCM does not second guess the information. If we were to be faced with this dilemma we would simply lick one finger, place it in the air and check to see if the temperature made sense.

This is not the way the PCM is designed to work. The PCM takes the information it is provided with as a true input and reacts accordingly with the data to operate the engine control system. Here is where the problem now surfaces for us since the computer accepts this information and reacts upon it, coming to a “correct” outcome when it is actually incorrect.

There are many sensors used for coolant temperature, air flow, engine speed, and component position. A defective crank sensor which is internally faulty or has the wrong air gap can cause a no start. It will not report that the engine is rotating, which in turn will cause the computer in most cases to turn the fuel pump off and inhibit firing of the spark plugs and fuel injectors. The crankshaft is the main engine shaft that the pistons are connected to so that the vertical motion of the piston is converted into rotary motion to propel the vehicle. Keep this in mind as well, since the sensor is only transmitting the information via an electrical signal that is susceptible to faulty wiring or connections at the component. Also, the computer’s connections can also affect the outcome.

The engine management systems uses sensors to monitor engine component movement, position, airflow, and emissions in order to provide a clean running engine with good fuel economy and performance. Another sensor critical for engine start is the camshaft sensor this sensor is used to tell the computer the position of the camshaft(s) in relationship to the crankshaft.

Remember that regardless of the design of the sensor, the overall result is for an electrical input to go to the PCM for processing. So if your engine turns over but does not start, you could be out of fuel or have an engine sensor issue. Check the gauge, observe the check engine or MIL (malfunction indicator light). If you have any issues getting your car started, have a certified mechanic inspect your vehicle and make any necessary repairs.