ford fiesta engine warning light

You clearly spoke with an idiot! :o) The engine malfunction light does usually mean that there's a duff sensor, but if you reset the ECU with a problem, the warning light will re-illuminate.

By plugging in a diagnostics computer to the Fiesta it can give you the fault code. Then you can figure out what's up with the engine, fix it, and the light will go out.

Even so you need to have it looked at. It may not be running fine but in your use, you may not notice a difference. If you do a lot of cold running, city trips and your oxygen sensor has failed, you probably won't notice the difference of it running extra rich - but it could knacker the car's cat.

It could be serious or it could be something simple and cheap. You can take it anywhere where they have an OBD-II diagnostics system (as a blatent plug, I have one as a private individual, because I'm nerdy like that:

Be aware that many places charge a diagnostics fee, somewhere between £25 and £40, which can be a rip off if all they say is "it's code ." Whilst there's never any certainty with such things - a single fault code is more an idea about the problem in some respects - some places charge the £40, replace the exact sensor that appears to be duff, that doesn't cure it, charge another £40, replace... so on and so forth. You need somebody with the experience of understanding what a certain fault code can mean to a certain engine...

You can also ask that if they do the repair, they'll rebate the diagnostics fee...