Daewoo Tacuma Check engine light

[MarkU]

I have a Dawoo Tacuma 2.0CDX, since passing 40,000miles the check engine light has come on every 1,500miles, I've now done 60,000 miles (very annoying and expensive). The ECU engine code has always been P0420- Low Cat efficiency. I have had both the o2 sensors changed, but it still keeps coming on. Someone told me it was caused by cheap (contaminated ) fuel, but I've been filling up at the expensive stations for the last 1,000miles. I have now been told it's the Cat converter, however I'm not convinced, The fault seems to regular, every 1,500miles. I have my suspisions that the ECU is to blame, came anyone help me??

[Mark U]

Car passed MOT emissions test on thursday, (guess the CAT's fine) however check engine light came on on saturday, only 100miles later. Could it be the fuel cap is faulty? I've been told that the petrol tank should be pressurized and that after running there should be hissing from the tank if the cap is removed, is this true? Mine didn't, it made no noise. Hope someone can help.