So a few weeks ago, my engine light appeared on my screen - orange light. I took it to the garage who told me one of the engine sensors was covered in gunk because I have been using cheap fuel. He gave me some engine cleaning fluid and told me to fill up and give it a good run with some premium fuel. I did this and the car ran until two weeks later, engine light came on again. Took it to the garage for a full service where they told me it needed a new Catalytic converter. I was not convinced so took it for a second opinion. They gave it an emmisions test and engine MOT and told me it was running fine and emissions weren't bad. I have the documents to prove this. The mechanic said he had looked at previous diagnostics and they have been wiped. 

My engine light has come on again after 500 miles. Any ideas? Could I just need a new o2 sensor? Can you get o2 sensors on their own? Do I need a new cat converter after all? 

 

The car has only done 65000 miles hence why I was a bit taken aback when they said it needed a new converter.

Car is booked in at the garage for tomorrow for another diagnostic with a garage that I trust so they should be able to help me but wonder if anyone else has anything I should be on the look out for.

If the car has been tested and the emissions are good then changing the O2 sensor will make the garage better off. Nothing more because there is obviously nothing wrong with it.

In answer to your question, yes you can buy O2 Sensors separately, the front and back ones are different from each other and they are not difficult to fit. The front one is likely to be more expensive than the rear one.

If the check engine light has come on again then the starting point is to have the codes read and then start the diagnostics from there depending on what codes are showing up. In most cases the fault codes only show the symptoms and that is where the skill of the Technician comes in to diagnose what is causing the problem, although in some cases a faulty component can be diagnosed from a fault code with a high degree of certainty.

Your Cat may be passed its best and need replacing, although if it is at the end of its life then something like an intermittent misfire or fuelling fault may have lead to its demise and this would need fixed or the same thing will happen again. That is something to look out for if the Cat is diagnosed as needing replacement.

The other thing to bear in mind is that the fault that you are getting now might not be related to the previous one.

  

Fault code readers are very cheap. They plug into the car and show the reason why the light came on in the form of a code, say P1234. These can be researched on  the net, (with varying degrees of success.) Those who really know what they are looking at can filter out the rubbish. Newcomers may not be so lucky. I regard such a device as essential with modern cars. The one favoured around here is the ELM327 from tunnelrat(£20). Pair that with a laptop running FORSCAN which is free and with a bit of study and practice you will save a lot of money. (I used mine to turn on accessories that Ford had fitted but only enable on more expensive models.)

Whilst I have a fault code reader and forscan I have not had to use it much. But some years ago I had a Ford escort mk6 that failed on emissions. I generally do my own repairs but did not have a fault code reader. I took it to a garage to put a reader on it. He showed me on the display what the voltage was from the oxygen sensor, he said it seemed too low. He also showed me how many milliseconds the injectors were opening for and he said it was longer than it should be. He said it seemed to be a faulty oxygen sensor but said he couldn’t guarantee that for 100% certain. I bought a new one and fitted it. Mot test pass after that. Before fitting it my comparison test was to put my face by exhaust and breath in. Before replacing sensor it felt like it would kill me quickly, after fitting new sensor it was like I could breath it in forever. I have not looked at forscan enough to know if it tells you the oxygen sensor voltage or how long the injectors are opening. But that info meant I went for the oxygen sensor and did not waste money on a catalytic converter

Like others have said, you need to find the specific code and come back. Whilst the code might not give you a definitive answer as to whether it's an o2 sensor or cat that's failed, it should narrow it down to either the pre or post-cat o2 sensor reading that's triggering it.