mysterious check engine+ service light fault !?

[Marwan Hashem]

Hello, i hope making a thread and explaining my issue actually work, i would be surprised.. Anyways, i have a Citroen c4 Picasso with the 1.6thp 165hp (ep6cdt 6 speed auto) i had an issue with the car it kept doing a weird fault. A check engine fault only appears in the car player screen and disappears immediately happens when slowing down for a speed bump, traffic or going to park the car but happens randomly not every time in a sitiuation like this, Also the check engine and service light come on the dash and the car doesn't start normally. So i took it to my mechanic he said there is multiple faults stored so he advised me to change the car's battery first before changing anything else. did that the issue still there then we started changing the spark plugs also still there then changed the throttle body because the accelerator was way too laggy and didn't accept to be reprogrammed and some issues with the accelerator got fixed after changing the throttle body but the main issue was still there, then he checked the timing chain kit was faulty so he changed it, the oil valve, VVT sensor and before all these issues i did an oil change so the oil is new Unfortunately tried out the car after putting money and changing all these things the car still gives both check engine and service light but we went on a test drive with the fault the car runs quite good and smoothly but when trying to hit boost and push the car a bit it starts to hesitate and cut feels like a fuel cut or something (ps : high and low fuel pressure pumps readings are okay and the turbo reading are okay)..I'm very bummed out about the car and i want to know where is the exact issue that is causing the fault to appear. sorry if the thread is a bit long and thanks for advance.

[xantia_v6]

It would help to know what fault codes are currently being generated.