I put a note up on another thread the other day - I had the warning light come on, cleared it, come on again about a week later, meanwhile the car was running a little better than usual (with a stumble/flat spot randomly between 2500-3000 revs). After a few cycles of this I got the car hooked up for a diagnosis to find my top lambdas were showing an error code.
before changing anything I was advised to check for any leaks in the air inlet pipe and to try swapping the MAF when that showed no improvement. Changing the MAF fixed it all as apparently it was throwing a way out mixture into the engine there was no way the lambdas could manage so they threw a error code.
Might be worth a check before replacing both lambdas then finding you need to spend more for a new MAF.