Citroen C3 engine malfunction and engine fault

In the last few months when taking a 3rd exit on a round about I’ll either get a yellow engine warning light with a ! In the centre telling me there’s an engine fault and to please repair.

On 2 instances now I’ve had it say engine malfunction with a red engine light and stop light lit up and all power lost to the car.

I’ve had my full service done only in September, I’ve had the error codes check which only read “immobiliser not unlocking”. I just had a new battery changed as the previous owner never did it. The battery was drained down to 3% by the time I changed it. I thought the battery could’ve been causing it due to how low it was but it’s once again happened today. I want to sell the car and change it but I’m unable to do so with these constant warnings. I’ve only just hit 73k miles on it, could you provide any insight to what may be causing it?