How To Test The 1990, 1991, 1992, 1993, and 1994 Lexus LS400 MAF Sensor

This tutorial will help you test the MAF sensor of the 1990, 1991, 1992, and 1994 Lexus LS400.

This MAF sensor can be easily and accurately tested with a multimeter that can read Hertz frequency.

This tutorial was originally written in 2009 and has been updated.

Puedes encontrar este tutorial en Español aquí: Cómo Probar El Sensor MAF De 1990-1994 Lexus LS400 (en: autotecnico-online.com).

Lexus LS400 MAF Sensor Testing Basics

This is a brief summary of the performance test of Lexus Mass Air Flow Sensor that you'll be performing in this article.

OK, you're gonna' start by checking the basics. These are the 5 Volts and battery Ground to the MAF sensor. After that, we'll test the actual performance of the MAF sensor as the engine is running. Use a digital multimeter for all tests where a multimeter is called for. This multimeter must be able to read Frequency Hertz (Hz).

The output signal of the MAF sensor is a low frequency Hertz signal to the ECM. The MAF sensor works this way: the frequency of the signal increases as the engine accelerates away from idle. This increase in Hz frequency is tied to the amount of air going into the engine. This means that as you accelerate the engine the Hz frequency reading on your multimeter should increase also. This increase in the frequency reading of the multimeter is immediate upon opening the throttle and it should be a steady reading at idle (after the engine has reached normal operating temperature). The readings should not flip flop around. Again, as soon as you open up the throttle the multimeter's reading should go up. At idle, the multimeter's reading should be steady for that specific RPM.

At idle you'll see the MAF sensor producing around 36.5 Hertz. This Hz signal at idle depends on several factors some of which are:

  1. Engine temperature. The colder the engine, the higher the idle and thus a higher Hz frequency signal. The closer the engine is to normal operating temperature at idle you'll see a lower Hz frequency signal.
  2. The actual idle of the engine. You can take two or three of the same car and park them side by side and all three will not idle at exactly the same RPMs due to various reasons.

When testing for this Hz frequency signal reading at idle the important thing to know is not an actual Hz frequency signal at a specific RPM, but to look for crazy fluctuations in the Hz frequency signal. For example: If at Idle the voltage reading starts to spike up and down without you accelerating the engine.

Now, as you accelerate the engine, the Hz frequency signal will go up. Again, you're looking for crazy fluctuations in the multimeter's Hz frequency signal. For example: You're accelerating the engine and the engine is accelerating and the multimeter all a sudden shows 10.5 Hertz (you'll notice a change in the engine performance at the moment the reading changes, too). This reading condemns the MAF sensor as being bad.

If you have access to an oscilloscope, I have included a scope waveform of what this MAF Signal should look like . Whether you use a multimeter or an oscilloscope, you'll be able to successfully diagnose this Lexus Mass Air Flow Sensor.

Lastly, these tests are not designed to diagnose an intermittent problem with the MAF sensor. They are designed to diagnose a hard fault with the sensor.

Circuit Descriptions Of The Lexus LS400 MAF Sensor

Circuit Descriptions Of The Lexus LS400 MAF Sensor. How To Test The 1990, 1991, 1992, 1993, and 1994 Lexus LS400 MAF Sensor

MAF Sensor on the 1990-1994 Lexus LS400 is two sensors in one. This is because the intake air temperature (IAT) sensor is part of the MAF sensor assembly.

Here's a brief description of the circuits (wires) of the MAF sensor:


1990-1994 Lexus LS400 MAF Sensor
Pin Wire Color Description
1 Brown (BRN) IAT Sensor Ground (from ECM)
2 Yellow-Black (YEL/BLK) IAT Sensor
3 Yellow-Green (YEL/GRN) MAF Signal
4 Blue-Red (BLU/RED) 5 Volts (from ECM)
5 Brown (BRN) Chassis Ground