Citroen C2 1.1 keeps cutting out when stopping or sometimes changing down to 2nd

More than likely you have a vacuum leak or even an idle air control issue.

However, let's start with checking for codes.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Unfortunately, we don't have a manual on this vehicle, but we do have a number of guides that will help solve this.

Please review each of these and let us know what you find with this.

https://www.2carpros.com/articles/idle-air-control-valve-service

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please run through this info and let us know what you find, and we can go from there.

Thanks