Faulty Sensor? How to test Automotive Sensors in 5 Easy Steps

Vehicle sensors detect chemical and physical changes in a car, transmitting the information to the computers.

But the sensors can sometimes fail and relay the wrong information- or none at all.

This affects the vehicle’s efficiency and performance. Safety, too, especially if the damaged sensor measures operations such as braking and steering.

Here, we explain the steps to test the different types of sensors used in cars when they show signs of failure.

Car Sensor Testing Procedures

The sensors used in cars are many and varied. Some of these common while others can only be found in luxury models.

The car sensors described below are the main ones- the type you’re likely to come across.

Materials and tools you will be needing:

• Digital volt ohm meter– this measures electrical faults which are often the cause of malfunction in a car sensor.
  
  For correct and reliable results, ensure the DVOM is accurate.
  
  For the basics on how to use the digital volt ohm meter, check out this YouTube video.

• Jumper wires and testing probes– you don’t want to damage insulation when testing leads and testing probes will come in handy. Jumper leads are also necessary for some tests.
  
  These are the main testing tools testing required but you can equip yourself with extra ones if possible. You may want to have an infrared thermometer with you or a scan tool to display testing data.
  
  For proper testing results, ensure the battery is in perfect condition- fully charged and its connections intact and routed correctly

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Now, on to the testing procedure itself.

How to Test Automotive Crankshaft or Camshaft Position Sensor

The crankshaft position sensor continuously measures the rotational speed and position of the crankshaft. This is important for the ECU to determine the timing of engine operations, such as fuel injection.

When the sensor fails, it can mean the engine performing poorly or even stalling.

Crankshaft or camshaft position sensors usually come with voltage or resistance specifications. Testing them for failure involves measuring voltage output and resistance then matching the results with those of the manufacturer.

This is how to test the sensor.

Step 1

Inspect the sensor for visual signs of damage such as worn insulation or loose and damaged connectors or mounting bolts.

Step 2

Set the digital volt ohm meter to read millivolts. Turn on the ignition without starting the engine.

Step 3

Connect the DVOM positive probe to one of the sensor connectors and the other probe to ground. The voltage should be within the manufacturer values, usually 200mv.

Step 4

To test for resistance, set the DVOM to Ohms scale. Unplug the sensor and connect either leads to the DVOM probes.

Step 5

Read the Ohm or resistance values and compare them to those of the manufacturer.

If they don’t match, the sensor is defective.

If the resistance is infinite, you have an open circuit within the sensor.

If the readings are zero Ohms, the sensor is shorted. Change it to avoid engine problems. 

How to Test Mass Air Flow Sensor

This sensor measures the amount of air entering the engine as well as its density.

Usually located in the area between the air filter and the throttle body, the MAF sensor helps the car computer to control engine performance. The signs that it has failed include misfires or the car stalling.

Test this sensor using these steps.

Step 1

Locate the sensor in the vehicle’s air intake duct. Inspect it for damage, dirt, or loose connections.

Step 2

To test if the sensor is receiving power, set your DVOM to 20V DC.

Unplug the sensor and connect the meter probes to the harness terminals- the negative to GND and the positive to B+.

The meter reading should indicate a value close to the battery voltage or from between 10 to 13 volts.

If not, the circuitry should be faulty.

Step 3

Reattach the sensor and identify the signal and power terminals. They will usually be indicated as SIG and GND respectively.

Connect the meter to these terminals.

Step 4

Switch on the ignition but do not start the engine.

Observe the readings, which should range 0.2 to 1.5 volts. Raise the engine speed.

The readings should change accordingly, reaching up to 2 volts.

Step 5

To check for resistance, set the DVOM read ohms, then unplug the sensor.

Connect one of the meter’s probe to the signal terminal of the MSF sensor and the other to ground.

The meter should read 0 ohms or close to that.

Infinite resistance indicates a burnt out sensor.

How to Test Automotive Oxygen Sensors

The oxygen sensor is located in the exhaust.

By detecting the amount of oxygen in the exhaust gases, the oxygen sensor provides the information needed to ensure engine efficiency.

The computer is able to control the amount of air for the right combustion ratios, which not only helps to enhance performance but also controls emissions.

Use this procedure to test the sensor.

Step 1

Remove the sensor and check it for bad connections or exposed wires.  

Step 2

Re-install the sensor and turn on the engine. Allow it to run for about five minutes then turn it off.

Step 3

Set the digital volt ohm meter to read millivolts.

Connect the positive terminal of the digital volt ohm meter to the sensor and the negative to ground. The values should be between 100-1 000 MV.

Step 4

Switch the engine on again and observe the meter reading. The values should now change quickly.

If the reading stays still and high up to 500 millivolts, allow the engine to warm up a little bit more.

If the readings do not change, the sensor is defective. You need to replace it.

Step 5

Locate the vacuum port and open it to create a vacuum leak.

If the vacuum is working correctly, the meter reading should go up in a vacuum leak and drop when you close the vacuum port. 

How to Test Throttle Position Sensor

The throttle position sensor, TPS, is a type of auto sensor that usually mounts on the throttle body, the TPS can fail due to various reasons.

It could be loose connections or damaged parts. Here is how to test the sensor.

Step 1

Locate the sensor. It’s found in the carburettor side in older models and fuel injection for newer vehicles.

Step 2

Check the sensor for loose connections or worn insulation.

Step 3

Set a digital volt ohm meter to measure 20 kilo Ohms.

Step 4

Attach the DVOM’s positive lead to the central terminal of the sensor and the negative to one of the remaining terminals.

Step 5

Open up and close the throttle opening slowly.

The reading on the digital volt ohm meter should decrease or increase depending on the terminal used. 

The reading should also be gradual. If unsteady or irregular, the TPS is faulty and needs to be changed.

How to Test Manifold Absolute Pressure Sensor

This sensor is usually located on the engine firewall.

It detects the outside and inside and outside the manifold. This helps the computer to calculate the fuel amounts needed by the engine. 

Here is how to test it.

Step 1

As with the other sensors, inspect the connector of the MAP for damage.

Step 2

Disconnect the sensor. Use a jumper wire to connect the sensor to the terminals.

Step 3

Switch on the ignition but don’t start the engine.

Step 4

Set the DVOM to 20V DC and connect its positive terminal to the B terminal on the sensor. Connect the negative terminal to ground. The DVOM should read between 4.5 volts to 5 volts.

Step 5

Start the engine and leave it to idle. Repeat step 4. If the DVOM readings do not change, the sensor is damaged and requires replacing.

Conclusion

Sensors can and do get faulty. When damage occurs, they need replacing.

Testing a sensor before you install a new one is essential.

It ensures you do not change a working one. When carrying out the tests, use reliable tools and equipment. You don’t want to get inaccurate results.

Most car sensors are expensive and wrong diagnostics can be costly. Some sensors might require testing procedures that are different from those described here.

When in doubt, always consult the manufacturer manual. Also, have a qualified person assess the sensor you suspect to be damaged.