Citroen C4 my key will unlock doors but car won't recognise it to start unless i take + off main battery then put it back on i've since put a new battery on and still same

Have you replaced the car battery or was it key fob battery ?

I've replaced the key batteries 1st and then the car battery..it worked fine for 2 days after putting new battery on but now back to not working

Try resynchronisation procedure - Switch off the ignition and take out the key - Wait 1 minute without pressing the buttons of any remote control - Place the key of the high frequency remote control to be synchronised in position "ignition+" - Within 10 seconds, press the locking button for 5 seconds - Switch off the ignition and take out the key - Repeat the procedure with the other keys from point N°3, otherwise move on to point N°7 - Wait 30 seconds, the remote controls are synchronised

Sorry forgot to say its a stop start as well

There are a few stop / start related fuses you may want to check Citroen C4 Picasso mk2 (from 2013) – fuse box diagram Year of production: 2013, 2014, 2015, 2016 Fuse box under dashboard The type of electrical of the vehicle depends on its level of equipment. To identify the type of electrical system on your vehicle, open the bonnet. TYPE 1: electrical system does not have any fuses in front of the battery TYPE 2: presence of an additional fuse box in front of the battery Type 1 fuse box Fuse box number 1 Fuse Ampere rating [A] Function F8 5 Steering mounted controls F18 20 Touch screen tablet, audio and navigation system, CD player, USB ports and auxiliary sockets (AUX) F16 15 Front 12V socket F15 15 Boot 12V socket F28 5 START/STOP button F30 15 Rear wiper F27 15 Front screen wash pump, rear screen wash pump F26 15 Horn F20 5 Airbags F21 5 Instrument panel F19 5 Rain and shunshine sensor F12 5 Keyless starting unit F2 5 Manual headlamp adjustment control Fuse box number 2 Fuse Ampere rating [A] Function F9 15 Rear 12V socket Type 2 fuse box Fuse Ampere rating [A] Function F3 3 START/STOP button F6A 15 Touch screen tablet, audio and navigation system, CD player, USB ports and auxiliary sockets (AUX) F8 5 Alarm F9 3 Steering mounted controls F19 5 Instrument panel F24 3 Rain and sunshine sensor F25 5 Airbags F33 3 Memorisation of diving position F34 5 Electric power steering F13 10 Front 12V socket F14 10 Boot 12V socket F16 3 Map reading lamps in row 1 courtesy lamps F27 5 Electronic gearbox gear selector F30 20 Rear wiper F38 3 Manual headlamp adjustment control Fuse box in engine compartment Type 1 fuse box Fuse Ampere rating [A] Function F18 10 Right hand main beam F19 10 Left hand main beam Type 2 fuse box Fusebox 1 The fuse box is place in the engine compartment near the battery (left-hand side). Fuse Ampere rating [A] Function F9 30 Motorised tailgate F19 25 HIFI amplifier F21 3 Hands-free starting reading unit Fusebox 2 An additional fuse box is fitted in front of the battery (fob type 2). Fuse Ampere rating [A] Function F19 30 Front wiper slow/fast speed F20 15 Front and rear screen wash pump F21 20 Headlamp wash pump WARNING: Terminal and harness assignments for individual connectors will vary depending on vehicle equipment level, model, and market..