This vehicle came to us in August with a O2 sensor fault which turned out to be a cracked exhaust manifold. The manifold/cat (Unipart) was replaced along with the front O2 sensor,it has now returned to us after 2 months with mil on and code P0420 cat efficiency below threshold. Gas test extremely good, no vac or exhaust leaks, blanked off egr, no change. Scoped rear sensor and after an initial warm up period its trace follows very closely that of the front one suggesting that the cat isn't working but the gas test says it is. Have checked for ECU updates with local dealer but nothing available. The engine runs ok albeit a bit gutless, any help would be most appreciated, cheers Dave.