Emission control system

Good to hear that it worked for you, but there is an element of luck here - sometimes it will work and sometimes it won't.

The "emissions light" is simply an indicator that there is, or has been, a fault somewhere in the engine management system - and there are lots of possible faults (i.e. lots of components that might go wrong, in various ways). The intention by the manufacturer is that when it lights up, you are meant to connect a diagnostic reader to the diagnostic port and it will give you more information about which component is faulty.

In some cases, the fault can be intermittent - e.g. an actuator will stick the first time you use it, and then will free up for a while. In that situation, clearing the fault code can help - the fault may not re-appear for some time, if ever.

But in other cases, the fault will be permanent - e.g. a wire breaks or a sensor goes open-circuit. In that case, clearing the fault code won't help - it will simply come back every time you turn on the ignition, because (obviously) clearing the code doesn't fix the underlying fault!

The one advantage of reading the codes before clearing them is that it can give you a bit of advance warning that a component is on the way out, which may be helpful if there are long lead times to get a replacement.

Here's an example of a list of possible fault codes for PSA vehicles, which was actually posted by an owner on MHF: [You must be registered and logged in to see this link.]

[And BTW, the advice on avoiding supermarket fuels is maybe a bit arbitrary too - all diesel fuels have additives.]