Citroen C2 rough idle with new O2 sensor

[OneRandomBX]

Well this is interesting. Drove around with the sensor connected.
Still runs rough in closed loop when cold
But, once the car warms up completely it switches to closed loop - and actually runs pretty well!
Short term trim was hovering around 0 at idle but long term was still at +28, although it would go down while driving.

The weird thing is, when at idle sensor 1 would swing between 0.350 and 0.650 while sensor 2 would swing between around 0.100 and 0.800
Same goes when holding the engine around 2k rpm

Make it make sense...

[PaulC5]

I may be wrong, but I thought sensor 2 is after the catalyst and is there to check the catalyst is working. With everything in good condition its values should not vary much. If it varies similar to the sensor before the catalyst it means the catalyst is not working and needs replacing. Since your sensor 1 may not be working the fuelling may be too rich and has affected the catalyst so it cannot cope.

[KenethMahone]

Hello everyone
I swapped the O2 sensor (the one before the catalytic converter) on my 2005 C2 1.4 petrol the other day.
It started fine but after driving it I noticed an intermittent issue.

The engine runs rough on idle and the revs go down when I give it some throttle, sometimes it even dies.
I can rev it up by giving it gas very slowly and when it goes above around 2k RPM it runs completely fine.
Shutting it off and starting it again usually makes it run normally but it starts acting up again after a while.

It's drivable but of course hard to drive in low revs and partial throttle.
I've disconnected the sensor and now it runs completely fine.

Do you have some ideas about what the issue could be?
Thanks!

Since your Citroen runs better with the sensor unplugged, the ECU is likely receiving incorrect data and struggling to adjust the fueling. In "Open Loop" mode (unplugged), the car ignores the sensor and uses safe, pre-set values, which is why your idle smooths out.

The intermittent stalling and rough low-rev performance suggest the new sensor might be incompatible or "lazy." Citroen 1.4 engines are famously sensitive to the resistance values of O2 sensors; if you used a universal or budget aftermarket part, it may be providing a signal that the ECU interprets as an extreme fuel imbalance. You should also try a BSI/ECU reset by disconnecting the battery for 20 minutes to clear the "auto-adaptives" (fuel trims) the car learned from your old, failing sensor. This forces the engine to start fresh with the new part's data.

If you decide to post photos of your sensor's part numbers or diagnostic codes on the French Car Forum, your phone might save them as WebP files. Since many enthusiasts use older forum software that can't display this format, you can use WEBPtoJPGhero to quickly flip them into high-quality JPEGs. This ensures the experts can see exactly what you’ve installed and help you diagnose the fault faster.

[ozvtr]

Still runs rough in closed loop when cold
But, once the car warms up completely it switches to closed loop

Do you mean open loop when cold? These two sentences are conflicting.
What temperature does the scanner say the MAP sensor and engine coolant sensor are when the engine is cold and not running? Do they agree? Do they agree with the actual air temperature?

Short term trim was hovering around 0 at idle but long term was still at +28, although it would go down while driving.

I'm guessing that the engine ECU is not divulging the long term fuel trims and the scanner results are bogus. So we can forget the LTFT results.

The weird thing is, when at idle sensor 1 would swing between 0.350 and 0.650 while sensor 2 would swing between around 0.100 and 0.800
Same goes when holding the engine around 2k rpm

These results are exactly the opposite of what they should be!!! With sensor2 oscillating that much you should be getting an aging catalyst error P0420!
These results seem to fly in the face of the results of your table on the previous page.

Conflicting results are making it very difficult to diagnose the fault.

[PaulC5]

Maybe we should all chip in and help to buy a new sensor from a dealer and not a cheaper copy part.