Sounds the best place to start - we're chasing rainbows on the oil until that's eliminated. A set of new plugs too maybe. Give it plenty of time to be hot before doing the emissions test. I've noticed the guy who does my MOTs doesn't do the emissions until about 20 mins into the test with the engine running right from the start.Now I've read the thread I think you guys are right with the Co2 sensor. My dads 2.8 had the same issue a few years back, once replaced the readings where much better because the Ecu then knew what adjustments to make.