I have the same problem, has been reset several times over the past year, seems fine on motorway runs, then as soon as you hit town, it comes back on. If it is the pre-cats then don't change the lamda's like I done as it's a waste of money, you could try some cataclean, but I beleive Bob.dent tried some and never made any change.
Ok, so to be clear for Mr Metro the issue is caused by
The pre cat collapsing or damage of some sort, possibly
Engine oil or some contamination from the engine,
Possibly from removing oil from a plug well or over
Filling the sump? Cats dont like oil and the breather
System is ment to burn excess oil through the engine
But excess can get through and bugger the cat. Or they
Just collapse with age, as i understand it.
What ever the cause they are not workin to a level
The sensor expects and causes a code. It is not a major
Problem as it is not said to cause an emissions issue
At mot, as the bigger cats still do the job.
The problem is with the light coming on and possiblly
Hiding other issues. How do you reset the light?