Help with Upstream and Downstream Oxygen sensor readings.

[andyr12321]

Hi,
I have a Citroen C4 2009 1.6 petrol, when I use Diagbox to monitor the oxygen sensors, the downstream sensor is showing a rich fuel mixture 735 mV, and the upstream is showing a lean fuel mixture 1386 mV.

Would this be caused by the upstream sensor being faulty, and falsely showing the air/fuel mixture to be weaker than it really is, so the engine would make the mixture richer, and the downstream sensor is showing the real air/fuel mixture.

A few days ago, I used a fuel additive, hoping it might clean the upstrean oxygen sensor, I drove the car for the recommended time. When I got home, I used Diagbox to see if it has helped, and for a few minutes the downstream sensor was staying around 450 mV, but then it increased to 700 mV and nearly 800 mV. It was only later that evening, that I thought maybe the reason why the downstream voltage was reading 450 mV might be because the downstream sensor was in closed loop for a few minutes and the engine was adjusting the mixture using the downstream readings, and so it was able to accurately set the mixture. Then when the downstream sensor went open loop, and the upstream sensor was in use the mixture was rich again.

Any advice on what might cause the rich downstream and lean upstream readings would be welcome.

Thanks,
Andy

[wheeler]

Firstly are there any running issues, fuel consumption issues or warning lights/messages? Are there any symptoms or are you just looking at live data?
The downstream sensor is there purely to monitor the cat is working correctly.
The upstream sensor (when the engine is warmed up, idling & on closed loop) should constantly fluctuate between 100mV - 800mV, is the upstream not moving?

[andyr12321]

Hi, thanks for the quick reply.
The main problem is the emissions are high. The car seems to run OK when it's warm, when it's cold it does stutter a bit. The car does mainly short trips, so it's hard to tell if the fuel consumption has changed much.There are no warning lights, for several months there was an engine warning light on, and Anti pollution warning displaying on the screen. I was told by a mechanic not to worry about the anti pollution warning.

I used diagbox, and found a camshaft sensor fault , i changed the sensor, and the anti pollution warning disappeared , I found out later that if the camshaft sensor isn't working the engine will default to a rich mixture. I got the emissions checked at a local garage, the emissions are still high, but the mechanic mentioned the upstream sensor could be the problem.

According to the information on diagbox, the type of sensor used on the upstream should fluctuate around 1500 mV. The times I have been looking at the readings, when the engine is warm and the revs are correct ( i think 700 rpm), the mV fluctuates from 1400 mV to 1460 mV. I will have a look again tonight at the live readings.

[Kees]

As WHEELER told, the sensor must fluctuate even between a few millivolts and more than 700 millivolts.
This is done for a correct working of the catalytic.
This needs oxygen to work properly, so with a lean mixture there is a surplus of oxygen and on a rich mixture the engine runs smoothly, this is why the sensor must gif this reading.
It seems this one is broken.
Greetings,
Kees.

[andyr12321]

Hi Kees, when you say, It seems this one is broken, are saying the catalytic converter is broken, or one of the oxygen sensors?
Thanks
Andy

[Kees]

No, for me it seems the sensor is broken.
Greetings,
Kees.

[wheeler]

According to the information on diagbox, the type of sensor used on the upstream should fluctuate around 1500 mV. The times I have been looking at the readings, when the engine is warm and the revs are correct ( i think 700 rpm), the mV fluctuates from 1400 mV to 1460 mV. I will have a look again tonight at the live readings.

Are you sure Diagbox states this? doesent sound right.

If it was a failed cat the upstream sensor should still be fluctuating between 100-800 mV as its supposed to, if the cat wasn't working the downstream sensor would be copying the upstream sensor.
Out of curiosity what does the live reading do if the upstream oxygen sensor is unplugged? I would expect to see it go to somewhere around the 450mV mark.

[andyr12321]

The upstream is a wideband sensor and downstream is a narrow band sensor, it's the same on my C3 Picasso. I looked at the readings on my C3 on Sunday, I remember the downstream was steady around 450 mV, but I can't remember if the upstream was fluctuating around 1500 mV.
On Haynes autofix, they describe the different types of sensor, and the wide band is 1500 mV.
Last night I was looking at videos explaining wideband sensors operation, and they showed the output is mA, and should fluctuate between positive and negative.
I plan on changing the upstream sensor, but I'm concerned that there are 2 types of wideband sensors, one with mV output, and one with mA output.
I entered the Number plate into a parts website and there is a Bosch oxygen sensor listed that I intend to buy. I will try and find a data sheet on the sensor to see if it's mV or mA. I also want to remove the sensor and take it with me when i buy the new sensor so I can compare the 2 sensors.

[wheeler]

Ahhh, is it a 6 wire sensor? If so then this may be different.
I've only ever seen oxygen sensors that read in Volts, never seen one reading Amps.