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Bosch LSU 4.2 Wideband Wiring

EFI Wiring Fundamentals

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Hey All,

I did some researching on this but didnt see much of an answer, other than "thats how it is."

I am wiring a bosch lsu 4.2 wideband to an aem infinity 508. The bosch wideband has 5 wires on a 6 plug connector. But there are 6 wires that go to the "ecu" side of the plug, and AEM says it has to be wired that way. Can anyone explain whats happening inside the plug that requires this extra wire? And am I able to switch out the bosch connector with a DTM replacement? And what would the steps be?

Here is AEM's wiring diagram:




G'day Noah. The 6th wire heading off to the ECU is connected to a resistor inside the o2 sensor connector that is laser trimmed at the factory for calibration purposes. The other side of this resistor is connected to the Vs wire, but inside the connector housing. This leaves only the other 5 wires to head down to the sensor body itself.

This is why its not a good idea to swap out the OEM connector for a DTM one. Some ECU's claim they are capable of getting a valid o2 sensor signal without the calibration, but it will require free air calibration.

G'd Day mate. Thanks for the quick response.

Would you mind going a little deeper into what is exactly happening on this 6th wire? Im imagining that the ecu is able to detect/read the resistance on that wire and get a value, and then that value is then used to determine what the values (on say the signal wire) coming from the wideband are?

Just wanna know how it works is all.



Yup, the ECU looks at the resistance on the wire and uses it as a calibration value. I assume it uses another resistor and a bridge to do this inside the ECU.

For the other five wires, two of then are for the heater, which is fed a 12V supply on one side, and the other side PWM controlled to ground. A good wideband controller will also look at the resistance of the heater circuit (via monitoring the current flowing in response to duty cycle) and get a measurement of the sensor temperature too, this way it can avoid overheating it and killing it.

The other three wires are the oxygen ion pump positive and negative, and the narrowband sensor cell voltage output. How they actually work, is that current is pulsed through the oxygen pump wires, this pumps oxygen into, or out of the narrowband oxygen sensor measurement cell (nernst cell), until the narrowband output reads stoich (450mV). When the narrowband cell reads stoich, the actual air/fuel ratio in the exhaust is relative to the amount of oxygen that needed to be removed or added to the measurement cell, which is directly proportional to the current passing through the oxygen pump.

The wideband interface runs a control loop which keeps the nernst cell output at 450mV by varying the current through the oxygen pump. The current through the oxygen pump is what is then measured to get the afr reading of the current exhaust gas sample :-).

The more air it has to pump, the further it is from stoich. The direction of pumping indicates whether it is rich or lean of stoich.

I have found this webpage useful for understanding O2 sensors:


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