Common Engine Sensors

Testing engine sensors to check functions is not that complicated.
If you have a VOM (Digital Volts/ohm meter) and some of the tips here you can do this easily.
In order to test most sensors you will back probe the connections for your readings.
Do not discard a sensor if you don’t get a reading without first making sure you have
probed the connection in far enough to make a connection. Check and re-check.

AFR sensor and
Oxygen Sensors = (O2)
Throttle Position Sensor = (TPS)
Mass Air Flow Sensor = (MAF)
Engine Coolant Temperature Sensor = (ECT)
Intake Air Temperature Sensor = (IAT)
Exhaust Gas Recirculation = (EGR)

Most Common Engine Sensors and understanding their purpose.

AFR sensor – Oxygen Sensors (O2)

Purpose: Measure the amount of oxygen left in the exhaust stream to provide feedback to the computer about whether the air/fuel mixture is rich or lean.

Poor fuel economy
Rich fuel mixture
Check Engine Light
Failed emissions test.

AFR Sensors: Link to article

Tip: Test the O2 sensors in front of catalytic converter.

With the engine warm, check the O2 sensor output with a VOM. Rev the engine to approximately 2000 RPM. A good sensor will oscillate from below 0.2 volts to above 0.8 volts. A bad sensor will be flat-lined and show no response to snapping the throttle. The front sensor will be oscillating actively. If the sensor is flat-lined at zero volts, it has failed.
Never replace any O2 sensor without testing it first.

Throttle Position Sensor (TPS)

Tells the computer what the throttle position is

Stumble or hesitation on acceleration

Test: With the key on, engine off, check the throttle position sensor output. Gradually press the accelerator to full throttle. The vom should ramp smoothly with no spikes or drop-outs.

Tip: Most Throttle Position Sensors wear out just off the idle position.

Mass Air Flow Sensor (MAF)

Measures how much air is flowing into the engine. There are a couple different styles of MAF sensors that have been employed over the years. Early versions were called Vane Air Flow (VAF) sensors. They had a spring loaded door that controlled a wiper arm across a resistive pad. There is a black plastic cover that, once removed, will allow access to this resistive circuit. You can shift the wiper arm to a clean spot on the resistive
circuit to extend the life of your VAF.

Poor fuel economy. misfires, stumbles, hesitation.

Tip: Check the MAF sensor voltage on your VOM. With engine at idle, a steady voltage should be visible on the screen. Tap lightly on the sensor housing. A good sensor will not react to the vibration. If the graph jumps, the sensor is bad and needs to be replaced. Next rev the engine and verify that the numbers climb smoothly in proportion to engine speed. A jump or slow-reacting screen indicates a failed or dirty sensor.

MAF related trouble codes are often caused by a leak in the air ducts leading to the sensor. If the MAF sensor is fouled it can often be cleaned with an aerosol electronics cleaner. Be very careful with this procedure as you can destroy a good sensor by being careless. Do Not touch the sensor wire. You have been warned!

Manifold Absolute Pressure = (MAP)

Measures mass of air entering the engine. Has a direct relationship to altitude, BAP (barometric absolute pressure) and is proportional to the air density. This is necessary to calculate air density and determine the engine’s air mass flow rate, which in turn is used to calculate the appropriate fuel flow. Several makes use the MAP sensor in OBD II applications to test the EGR valve for functionality. Not all makes will have this sensor. Some will use the MAF.

Poor fuel economy. misfires, stumbles, hesitation, very black exhaust.

Tip: Check the MAP with your VOM. A high vacuum reading will give a low voltage signal. A low vacuum reading will give a high voltage signal. Low vacuum means the engine is under load and needs lots of fuel. Look at it this way, low signal voltage, low fuel requirements. High signal voltage, high fuel requirements.

Engine Coolant Temperature Sensor (ECT)

Tells the computer if the engine is warmed up. Some control the electric fan.


Poor fuel economy.
Hard Starting in Cold Weather.
Poor performance.

Tip: After sitting overnight, the Engine Coolant Temperature should equal the air temperature. As the engine warms, it will rise steadily to 200 to 230 degrees Fahrenheit. Most sensors have a resistance table to go by in the service manual. Temperature = so much resistance. No match then replace sensor.

Intake Air Temperature Sensor (IAT)

Purpose: Tells the computer the temperature of the incoming air used to calculate the air density and fine-tune the air/fuel mixture.

Symptoms: Poor economy, stalling or rough idle when cold.

Tip: The Intake Air Temperature Sensor will read approximately equal to the outdoor temperature when the vehicle is moving. Most common cause of trouble codes related to the IAT are a bad connection, or a sensor left disconnected after servicing the air filter. Test the same as ECT above.

Exhaust Gas Recirculation (EGR)

While the EGR doesn?t typically have its own sensor, the system is important and a
common cause of Check Engine Light problems. The EGR system mixes a controlled amount of exhaust gas with the intake air to properly control the combustion process.

Failed emissions test, detonation (pinging) under acceleration. Poor or Rough idle.

Tip: If the EGR valve is vacuum controlled, With engine at idle, manually open the EGR valve (do not manually move an electrically driven EGR valve) and listen for a change in the engine. If there is no reaction, the EGR is leaking or passageways or the valve itself are likely to be clogged and need to be cleaned.

Tip: Need information or service procedures for your vehicle? Try Autozone online as they have many vehicles listed and manuals on line for free at, click on Repair Info on left side, then next page click on Vehicle Repair Guides on picture with red car.

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This post was written by Michael on October 19, 2008

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