Hi, I've got a skoda fabia from 2003 which has been given me an engine management light on and off for the past few months. And I could use some advise on hot to pinpoint the exact problem.

Short history, beginning of december the car gave me an P0170 & P0138 fault, Fuel trim bank 1 and 02 Sensor circuit high voltage bank 1 Sensor 2. I noticed no other symptoms (fuel consumption, power, temperature were normal) so I deleted the error and figured if it would come back I would take a look at it. Didn't see anything untill I was driving home for Christmas and the light popped back on, gave me a P0170 and a P0302 (Cylinder 2 misfire) before dying on me. Turns out the coil in the 2nd cylinder shortcut and blew the main fuse. Roadside assistance had a spare one and helped me on the road again, I also discussed the earlier error codes and he said it was likely due to the coil failing and I shouldn't worry.

Everything seemed fine until a few days later when the P0170, P0130 (Sensor Circuit Malfunction (Bank 1, Sensor 1), and P0420 (Catalyst system efficiency below threshold, bank 1) light popped up. Again no symptoms in driving. Visited a garage who didn't have time for extensive diagnostics who told me it could be anything from my car not being able to handle the new E10 petrol (10% bio-ethanol) introduced in the EU to my sensors/catalytic converter having broken down. Decided to put some additive in the fuel and stop refueling with E10. The garage deleted the errors to see whether they would come back. A few days later when I wanted to drive back home (1000km trip) the light came back on and it was P0138 again. I visited a skoda dealer and a few garages to discuss whether it was a good/bad idea to drive back without solving the error first. Everyone told me you'll be fine and the light turned off by itself before I visited the Skoda dealer and didn't turn back on for the 1000km trip back home. 

Few weeks later the light turned on with the P0138 again and turned off by itself a few days later, few weeks later the light turned on with a P0138 and a P0170 and turned off again by itself a few days later. Week later it turned on again now with a P0138, P0170, P0420 code. 

So my conclusion so far is that there is definitly something going on however I don't know yet what exactly. After replacing the coil I've seen a O2 sensor malfunction in Sensor 1 once, and several times in sensor 2. I've also seen the fault code suggesting the catalytic converter is under performing but from what I gather on the web is that the 02 sensors are used to determine the performance of the converter. So either 1 or 2 sensors need to be replaced or the catalytic converter needs to be replaced. Additional info, my simple OBD-II reader shows that the O2B1S2 voltage is cycling consistently. 

Is there anyway for myself to figure out which one is the problem, or should I consider other causes?

Have you checked the brake servo vacuum hose for cracks and leaks?

No I have not, I will put it on my list to check. Haven't felt anything weird going on with the brakes though, they still turn stiff when i pump them without the engine on. 
Would you be able to elaborate on how a leak in the brake vacuum hose would lead to these error codes?

They probably won’t to be honest. But as it’s such a common fault and a really easy check it’s worthwhile. 

1 hour ago, Hanky said:

Would you be able to elaborate on how a leak in the brake vacuum hose would lead to these error codes?

 

That would take longer and be less useful than you doing the check and taping up any big splits you might find.

Update: I Just checked mine and as far as I can tell the vacuum hose is perfectly fine, Not even the slightest hint of crack initiation. 

I found my original paper documenting the error codes and noticed that I had P0130 twice and the P0138 twice instead of the P0130 once and the P0138 three times what I reported initially.
So if it isn't the vacuum hoses how could I best pin point the root cause of this problem, O2 sensor 1, O2 sensor 2 or a Cat replacement?