Hello everyone,

I have a mercedes 190e 1988 2.0, and i have a idle problem which i suspect is from the O2 sensor. When the car is cold it idles about 1000rpm, but when it warms up the idle goes to 1500-1700rpm.


I've been struggling to read the duty cycle of my car to read error codes so i can fix the idle problem. So when i connect a multimeter positive to pin 3 and negative on pin 2, on the 8 pin diagnostic port on my left fender, i read less than 0.1V. Neither when the engine is on nor engine off/ignition on.

Tonight i figured out that when i connect pin 1 and 3 with a jumper wire and then measure i get 50% duty cycle, also keep in mind im not using a multimeter with duty cycle i measure votage and use simple formula i found on the forum:
1 - (voltage of pin 3/voltage of battery) x100

My questions are why do i need to connect pin 1 and 3 to get duty cycle, and why do i not get any error code when i disconnect the O2 sensor.

Also nothing changes when i dissconnect the o2 signal wire under the passenger seat, the car runs the same way with and without o2 sensor. When i measure the voltage of the signal wire i get 0.03V which is far from normal for a warmed up sensor.

I'm about to buy a new lambda sensor but i wanna make sure that the old one is faulty.

I would appreciate any ideas for how to test the sensor and how to get a propper duty cycle measure.