Front Oxygen Sensor replaced for emissions M.O.T , car ran fine before cold or warm, now car runs fine when warm but rough idles when cold.

Don't understand why the change when only replacing the oxygen sensor?

Any help would be greatly Appreciated

 

Thanks

Have you checked for any codes or seen a live trace of the data from the sensors?

O2 sensor shouldn't change the engine management when cold and in open loop unless it's generating odd readings. Have you tried disconnecting the new sensor, cleaning the terminals and reconnecting? Check for any vacuum leaks nearby, something might have been dislodged, though it might cause some idle hunting issues or other symptoms even when warm.

Memory is a little foggy with O2 sensors, does anyone know if you can rule it out by disconnecting it for a cold start?

What was the reason for changing the sensor?