Yeah man, had a scare today.
I was at my mate's house, started it up and the EM light was off! The car suddenly had more power, so clearly it was on limp mode with the 02 sensor issue.
Anyway I got home and when I next started it up it only did 3 cylinders. Looked at the spark plugs to see if I could divine the issue - they're pretty brown/black/burnt looking, some white powdery deposits there. Sorta rubbed that off and hoped for the best, it started up on 4 cylinders this time but the engine management light's on again.
I've unhooked the battery, hope it will reset the ECU which should help. Tomorrow I'll go buy some new spark plugs that are nice and clean and it should be plain sailing from there.