The 1.5 BlueHDi (Euro 6.2 / 6.3) engines are fitted with an SCR emissions control system. The SCR is a Selective Catalytic Reduction technique to reduce exhaust emissions of NOx (Nitrogen Oxides).
The SCR additive can also be named Adblue or Diesel Engine Fluid (DEF). There is a dedicated Adblue tank on the Citroen C3 BlueHDi models that need to be refilled with Adblue/DEF at regular intervals, dependent on the mileage travelled.
The SCR system is required to meet Euro 6.2/6.3 emission standards and a failure of the SCR system is enough to disable the engine from running (and polluting above Euro 6.2 levels).
Warning messages related to SCR system fault monitoring and Adblue levels come in the form of words and symbols on the dashboard.
Emissions fault: Starting prevented in X miles
NO START IN X miles
Are shown as a count down to engine starting inhibiting
The advice for NO START IN X miles or Emissions fault: Starting prevented in X miles is to switch off the engine and start it again to monitor the recurrence of the fault message. If the problem was only temporary then when the engine was switched off, the fault was reset. Drive the car again and no message would indicate the fault has cleared.
If the messages return, you have a fault with SCR and it needs to be diagnosed and repaired before the X miles runs out and the car won't start.
Whenever the ignition is switched on, the "Emissions fault: Starting prevented" or "NO START IN" message is displayed.
To be able to restart the engine, contact a CITROËN dealer or a qualified workshop.
The Adblue warning lights are explained here