Honda Repair: Honda civic D14 idle problem, lambda sensor, lamdba

Question
Craig,
I own a 2000 x reg civic EJ i think ( last of old shape three door hatch) 1.4 engine.  A month ago I had a new mid and back section of exhaust fitted.  Yesterday my engine emission warning light ( yellow on dash) came on and statyed on.  I noticed the car would idle as low as 150 revs then go up a little before resting below the normal ( 800 - 900 rpm level).  I recon the fault is either something to do with my exhaust ( maybee come loose) or my Lambda sensor.  I am having the exhaust checked for fitment as it has bene rattling about a bit.  What do you suggest might be the reasons behind my idle problem and fault light - am I looking in the right place?  The spark plugs/airfilter are all fairly new. It hasnt had an oil change for a while ( 12 months and 12,000miles).  Any advice would be most appreciated as this will drive me crazy. - I new I should never have sold my x reg 1.6vti 3dr, god I miss her!.  Cheers Craig.

Answer
I would want to know what code has been set with the warning light.  Its possible its the lamdba sensor but there are a lot of other things that can set that light off as well.  You will need to know what the code is.  Probably not related to the exhaust as there isn't any sensor on the exhaust system to test that itself.