Hi
I have a 56 plate 2.0 diesel which I bought 2 weeks ago. The guy I got it off said it had an oil cooler problem which seemed true as it had oil in the header tank but no water in the oil system. I read about putting 2 loops in the system which I've done by cutting the water pipes and putting in 2 copper pipe bends then I flushed the system and all seems ok. However when I refilled the coolant and started the car to get the water around the system I noticed the yellow plug light was flashing and it was blowing smoke out of the exhaust so I double checked everything as I had removed the airbox and had unplugged something off the pipe. I refitted it all again and it was fine the light had gone out. I just went out in the car and it's done it again however it's cleared again since. I drove the car 100 miles home and it was fine.
Any ideas.....

 

Might be EGR valve. I think yours has the Electronic one rather than mechanical. On front left side as you are standing in front of engine bay look just under extreme left front edge of engine cover if there is a wired plug then you have an EGR. Find a mechanic with a diagnostics computer and if there is a fault then it should come up. Typical symptoms are engine warning light or coil light or both come on and there is loss of power. If you turn the engine off and after a couple of mins restart, you may find the warnings go off and your power is back. But it will re-occur. You will also see smoke. You can try using oven cleaner to clean the valve but do it sooner rather than later.