The scrap I bought post ulez had an engine management light on.
2 garages were unable to find the fault.
I, a diy basic mechanic only able to do basic stuff, open the bonnet and found a coil cable that needed pushed properly onto the spark plug head.
Light gone and never returned.
These garages get paid and can't diagnose a simple fault.
Step g/son bought a similar car, with an occasional misfire and EML coming on. I never even resorted to diagnostics to track it down, I simply had him swap the ignition modules. Changing the modules, changed the misfire enough to prove it was a module fault - getting one from a scrap car, and fitting it to each cylinder in turn, proved which one was faulty.