What Sensors Can Cause a Car not to Start? Short Guide

When your car doesn't start and you have already checked every possible reason, one of the possible options to go for is the set of engine sensors. When they malfunction, it's pretty hard to locate the problem. Because the faulty sensor may look exactly the same as the normal one.

Overall, the car may not start when the following sensors malfunction: crankshaft and camshaft position sensors, throttle position sensor, mass airflow sensor, fuel pressure sensor, and manifold absolute pressure sensor. Also, in modern cars other sensors can prevent the engine from starting. For example, the oil pressure sensor with wrong readings may block the engine's operation to save the engine's life.

Also, you may not be able to locate the problem because the scanner will only detect some of these sensors' problems after the engine starts. As it doesn't start, you can't detect the problems.

Recently, I posted an article about fuses that can cause the no-start issue. You may want to have a look!

Here's what happens when sensors malfunction

Modern engines have dozens of different sensors. Some may not allow the engine to start while some others may make it stall after several seconds of working. Unfortunately, there are almost no specific symptoms that you can use to understand quickly which exactly the sensor is malfunctioning.

Let's go through all the sensors and their tasks to see why they can trigger some starting problems in your car.

1) Camshaft position sensor

For your vehicle to fire the spark and inject the fuel at the proper time, the engine management system needs to understand where the crankshaft is right now. It gives the ECU all the information about the position of the pistons.

If the sensor doesn't send the proper readings, the injection process and also spark firing will happen randomly. So, the engine will not start or it will stall if it's already working. You may hear some knocking and harsh work before it stalls.

2) Camshaft position sensor

Like the crankshaft sensor, this one sends data about the position of a certain camshaft (intake or exhaust). Thanks to these readings, the ECU understands where each valve is positioned right now. It helps it decide exactly when the injector should spray in some fuel and the spark should fire.

Also, thanks to these readings, the ECU makes sure that the engine works well and the timing is aligned perfectly.

3) Throttle position sensor

This one is extremely important. While the vehicle may start with the faulty throttle position sensor, it will not be able to work. This sensor sends the information to the ECU about the position of the throttle flap. Then, the ECU may instruct the throttle to let in more or less air, according to the situation.

Throttle position sensors work together with the Idel Air Control valve - IAC. This is yet another part that may cause immediate stalling after the engine starts or too low (too high) revs when the engine is idling.

4) Fuel pressure sensor

This is important for cars with direct injection. The fuel pressure sensor monitors the pressure in the rail and the ECU gets the information from it to control the fuel pump's operation. If the readings are wrong, the ECU may wrongly command the pump to stop pumping fuel. This may lead to stalling or not starting.

This won't work with older cars that have simple port injection and a simple single fuel pump in the tank. But even in these vehicles, fuel pressure sensors may cause bad deviations in the way the engine works.

5) Manifold air pressure sensor

In some vehicles, this sensor monitors the air pressure in the intake manifold. The ECU relies on the data from this sensor to make adjustments to the ignition and fuel injection. If the readings are wrong or absent, the ECU may cut the fuel supply and/or stop firing the spark plugs.

Not all cars have MAP sensors, but if yours is equipped with one, you should check it when your vehicle doesn't start.

6) Mass airflow sensor

This one is usually located on the pipe that connects the air filter box with the throttle body. This sensor monitors the amount of air supplied to the engine. Based on these readings, the ECU decides how much fuel it needs to inject into each cylinder to prepare the proper air-fuel mixture.

Although this is a cheap and easy-to-replace sensor, it's extremely important for the smooth work of the engine. It can lead to no-start issues and can also make your car stall.

How to locate the malfunctioning sensors?

Now, the most interesting part. First of all, you need to check other possible issues that may cause the no-start problem.

I'm talking about fuel injection, ignition, and compression. If you are sure that all of these factors in your engine are OK, you may start thinking about sensors and fuses.

Also, make sure that the following things are in order:

• the car has fuel in its tank;
• nothing blocks the air intake;
• the key is correct;
• there are no warning messages when you are trying to start the car;
• the start motor cranks when you turn the key or push the button.

So, the sensors may only be the culprits if everything else is working perfectly. The engine cranks but doesn't start.

Next, you may want to connect an OBD2 scanner to your car and try reading some codes. You may try reading them with just the ignition on or catching the codes when the engine is cranking. Sometimes, this will help you locate the problem.

If you can't see the reason why the engine doesn't start, you may want to check some sensors that you have access to.

Usually, it's all about the MAF sensor, throttle position sensor, and IAC valve. Other sensors are hard to locate and they will require some knowledge and maybe even special tools to be checked. In this case, you should take the car in for an inspection.

Can you replace the sensors on your own?

Well, if you perfectly know what sensor malfunctions, you can try and replace it on your own. But you should know that the electrical system of your car is now extremely complicated and you must know what you are doing.

Otherwise, you may fry something just by connecting it wrongly to the network. It's not uncommon for DIY repairs to hook up the sensor to the wrong wire and eventually end up needing serious and expensive repairs.

So, here are some precautions:

• make sure you know which sensor is malfunctioning - replacing them hoping for luck is a bad strategy;
• secondly, buy the proper OEM sensor - don't even try getting a cheaper aftermarket option, they don't last;
• watch a good manual or talk to a specialist to get some advice on how to install the new sensor;
• make sure you disconnect the battery before doing anything with the sensors;
• install and connect the new sensor according to the manual and make sure everything is secure;
• connect the battery and try starting the engine, see if this helps.

This won't work with all sensors because some of them will require a lot of work. For example, to replace the crankshaft position sensor in some vehicles, you will need to take apart almost everything under the hood.

Before you decide to do any DIY work with your car, watch a good tutorial video and try to fairly understand if you can do this and have all the required tools for the job.

My advice for all newer cars (up to 7-10 years old) - just find a good specialist who will do the job for you. This is faster and, in some cases, even cheaper given you save your time and don't risk breaking something in the process.

Can you somehow bypass the broken sensor?

While it sometimes was possible in old cars, the newer vehicles will not start unless all the listed above sensors work properly. You can't bypass any of them and need to make sure that all of them work well.

There is one thing that can help you start the car sometime: disconnect the battery for several minutes. The thing is that sensors may develop glitches. And when they do so, you can't start the car because they send the wrong readings to the vehicle's ECU.

Sometimes, you can "reset" them by disconnecting the battery. When you do this, the sensor turns off completely because it doesn't get the power. After you connect the battery, the sensor may work well for some time and let you start the car.

While this trick won't work in all cases, it's worth trying when you are stranded in the middle of nowhere and have no other choice but to call an expensive tow truck.