I have a 2016 Ford Titanium 125 which I have owned for 3 years. Occasionally upon startup the "Check Engine Now/Active City Stop is Not Available" would appear, but turning it off and again cleared it. Now since the MOT, the error messages appear constantly. If both lights appear, the car will drive but in fits and starts. If I turn the car off and on again, the Check Engine Now stays lit but the Active City Stop light clears and the car runs smoothly. The outside of the car has been professionally washed so I don't think anything is blocking the sensors out there. No other issues of note have arisen.

So, what am I looking at? Something potentially simple or complicated? Something that can only be addressed at a Ford service centre? How expensive might it run me?

Check engine now means there will be a fault logged. You need to get the codes read by something that can read both buses and all Ford specific codes. Forscan is a good tool if you have a laptop to do it yourself. 

1 hour ago, alexp999 said:

Check engine now means there will be a fault logged. You need to get the codes read by something that can read both buses and all Ford specific codes. Forscan is a good tool if you have a laptop to do it yourself. 

And if I download Forscan and, somehow, manage to figure out how to use it (it looks complicated as heck, will it help me reset the system?

31 minutes ago, Deggsy said:

will it help me reset the system?

No, but it may well tell you what the fault is and then you will be able to replace the broken items as required.