My SLC2 Free 2 always measures a lean mixture


I’ve bought the SLC Free 2 controller with the corresponding LSU4.9 sensor and done all the wiring and soldering.

I tried to test the system using the test with free air, with the lighter gas and another test but results do not seem right :

  • Free Air Test : Directly after powering the board, the screen shows a lambda = 1 (initialisation) and the temperature shown is already close to the operating temperature (~780°C). After about 5 seconds, the lambda value converges to the saturating lean mixture value of 1.36 (5V on linear output and 0V on the simulated narrowband output). This test looks to be good. However, the initial temperature shown is already close to 780°C directly after powering the board, is it normal although the internal heating element is initially cold ?

  • Sensor test with the lighter gas : Again, the test starts in free air, the behavior explained above occurs. After waiting about 1-2 minutes to ensure the correct settling time, I approach gas from the cigarette lighter but nothing changes, the lambda value still satures at 1.36 (while the temperature remains constant about 780°C).

  • Another test has been done in the exhaust gas of burner. I wanted to make sure that the measuring error was not caused by not being in a hot enough environment. The combustion gas had a temperature close to 350-400°C. Again the 1.36 value does not change although the combustion gas has a low enough O2-concentration (about 14-15%).

Extra infos:

  • The 0 Ohm resistor has been soldered to short both GND.
  • Continuity tests between the LSU4.9 sensor and the board have already been done and the connections look okay.

Do you have any idea where this measurement problem can come from ?
Thanks in advance !



Hello again,

After playing with the sensor, I figured out that the lambda value changes when I litterally put the sensor in the flame of the lighter. I can therefore reach a 0.68 value and by moving the flame very slightly, I can vary between lambda = 0.68 and 1.36.
This shows that the sensor itself seems to behave as expected.

However, its use in the exhaust gas of a burner (O2-concentration of 13.8% measured by an external device) remains unsuccessful because the lambda value is still 1.36.

Do you know if there is any unfavourable conditions that prevent the sensor from working properly ? For example, the flow of the gas ? Or anything else that we should take care of ?
Why do I have those differences in the lambda value determination between sensing the flame of a lighter and the combustion gas of a burner ?

Again, thanks in advance!


1.36 lambda is approximately 5.2% O2 concentration.