Hoping someone can help with some advice before I have to pay through the nose. My Focus has re-started a really bad rough idle fault. Essentially, it runs perfectly well, except while stuck in traffic. It intermittently (annoyingly it is not always repeatable) rev's upto about 1200, and down below 800. Sometimes the engine stalls. I had the same problem last summer, the engine management light came on - Oxygen Sensor was responsible. The light then switched itself off, and never came back on.
I switched fuel and the problem disappeared. However, as soon as the weather turned warmer after early spring, the problem has come back again. Any obvious ideas? Is it the oxygen sensor? Would the outside air-temp affect this? Is it likely to damage the CAT?
Any help would be hugely appreciated.