someone is cleaning the mass flow sensor

Figure 1: Mass flow sensor cleaning.

The mass flow sensor, or air flow sensor is one of the important sensors of the electronic fuel injection engine. It is installed between the air filter and the intake hose to measure the intake volume of the engine cylinder and adjust the fuel ratio according to it.

The mass flow sensor converts the inhaled air flow into electrical signals and sends them to the electronic control unit (ECU). As one of the basic signals to determine the fuel injection, it is a sensor that measures the air flow of the suction engine. So if you want to test it, how should you do? Let’s start it.





Test the Overlay Wire

Multimeter calibration.

Then you should pull out the plug of the mass flow sensor.

You should adjust the multimeter to the buzzer, the black pen to the car body with overlay, and the red pen to measure 5 terminals. If one terminal is connected and the resistance value is 1 Ω below, it can be judged as the overlay wire.

overlay wire test

Figure 2: The worker is testing the overlay wire.

Power Cable Detection

12V Power Cable

You should turn on the ignition switch and set the multimeter to 60V. The black pen is connected with the known overlay wire, and the red pen is used to measure the 12V power from the other 4 terminals, which can be judged as the power supply from the oil pump relay.

5V Reference Voltage

The black pen is overlaid with known overlay wire, the red pen to test the other three terminals which of 5V electricity, and it can be judged as ECU for 5V reference voltage.

Signal Wire Detection

You should plug back in its control plug and start the engine until running.

Then you use the probe to insert the control plug, and the black pen to contact the probe with the known iron line, the red pen to probe the last two wires.

The signal voltage between the signal line and the rail is about 1.5V when the engine is idle,
which increases with the increase of the air intake flow of the throttle pedal amplitude. Rapid acceleration should reach 3.5V~4.5V.

According to the increase of air intake, the multimeter voltage also increases. If there is an incoming temperature signal, the voltage will rise as the temperature rises.

signal wire detection

Figure 3: The worker is detecting the signal wire.


You should turn off the ignition switch when measuring the resistance to avoid damage to the multimeter.

You should be careful when starting the engine.

When using the probe, you can not make the two needles together to avoid a short circuit.

Inspection Methods of the Mass Flow Sensor

Once the mass flow sensor is failed, the engine performance will be affected. What are the faults of the mass flow sensor?

a black mass flow sensor

Figure 4: A mass flow sensor.

The damage of the mass flow sensor is often caused by the fact that the air filter is not replaced for a long time or the bad quality, which leads to the accumulation of dust in the mass flow sensor, thus affecting its detection accuracy and life.

There are five common faults of the mass flow sensor, including abnormal air flow voltage range, high air flow voltage, low air flow voltage, thin mixture, and thick mixture. All these faults will lead to the car idle shake, flameout, weak acceleration, high fuel consumption and dark smoke from the exhaust pipe.

Detection Methods

It can be judged by measuring the output voltage when the engine is running. The standard voltage is 0.8-1.4 V in the case of dynamic signal voltage at the end of the plug and idle speed of the engine. When it is accelerated to full load, the voltage signal can approach 4V. If it is not within the range, the mass flow sensor may be damaged, or it may be dirty, and you should clean it.

It can be determined by measuring the output voltage of the sensor. By a multimeter, you can get the general voltage value that is 5V, with a dryer used to the sensor. If the voltage changes with the size of the air volume, it would be normal. But if there is no change, it will indicate that the mass flow sensor fails and needs to be replaced.

air flow direction to the mass flow sensor

Figure 5: The air flows in the sensor.

For instance, a car is idle and unstable, is of bad acceleration. And it is suspected that the hot film mass flow sensor signal is failed. You can unplug the mass flow sensor when the engine is running and observe the change of the engine. There will be the following three circumstances.

Failure Solved

It shows that the signal of the mass flow sensor has deviated and there is no damage. The ECU has been controlling the oil injection according to the wrong signal. Due to the mixing ratio imbalance, engine combustion is abnormal, and there will be unstable engine speed or poor power.

When you unplug the mass flow sensor, the ECU cannot detect the intake signal and immediately enters the failsafe function. It will replace the mass flow sensor signal with the throttle position sensor signal so that the engine continues to operate at the replacement value.

Therefore, if the fault disappears when you pull out the plug of the mass flow sensor, it is explained that the signal is not correct before pulling out the plug, and the signal is right after pulling out the plug. It can be confirmed that there is a problem with the mass flow sensor and the signal voltage is generally low.

Bad parts quality -- you should replace them.
Dirt -- you should clean it with the agent.

No Change of Failure

It indicates that the mass flow sensor has been damaged or the circuit is bad, causing the ECU not to receive the signal at all or receive the overvalue signal. The ECU confirms the bad signal of the mass flow sensor and enters the failure protection function.

At the same time, the fault code is stored in the memory, and the failure lamp flashes (if there is). At this point, pulling out the plug of the mass flow sensor or not will lead to the same result, and the fault phenomenon will not change. Hence, there should be other causes. When the real cause is found, you must replace the mass flow sensor.

Slight Change of Fault

It shows that the mass flow sensor is good. Before pulling out the plug of the mass flow sensor, the ECU controls it according to the signal of the mass flow sensor. And the fuel injection volume is accurate and the engine is in good condition.

When you unplug the mass flow sensor, the ECU controls it according to the throttle position sensor signal, and the fuel injection volume is different. This slight change can be read from the data stream. Also, the engine condition is relatively not good.

Related Info

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How to Test a Crankshaft Position Sensor?