16804- catalyst system: Bank 1: Efficiency below threshold

Hello, I am a newby and am having real problems with getting to the bottom of a problem. My Engine warning light came on about a month ago and i took it to an audi dealership who said it was a problem with either the catalyst or lambda sensor. Opting for what seemed like the most probable cause i had the lambda sensor (pre cat) replaced. The car drove really well for 3-4days then the warning light appeared again!
Lookig on the forums here i decided to buy one of the vagcom diagnostic kits that plug into your laptop and i got the following fault code:

16804- catalyst system: Bank 1: Efficiency below threshold

This was the same fault code the dealership told me BEFORE the lambda sensor was replaced and now i am getting it again.It turns out the previous owner had changed the catalyst not long ago so the likelyhood of it being that is reduced i guess. The car itself has been really playing up...lack of power,no pull under acceleration, hesitating under acceleration and i think it may be getting worse.

Please does anyone have any ideas?? I'm lost!! Thanks

Hi EmmyJ, welcome to the A2oc
The Ross Tech web site usually gives a good overview of most fault codes - http://wiki.ross-tech.com/index.php/16804/P0420/001056
I run a diesel so have no first hand experience on petrol engine problems but as a starting point would check for leaks in the exhaust system from the manifold to the post cat sensor.

Cheers Spike

Welcome!
Sorry to hear of your problems. First up though congratulations on buying VAGCOM, that's a great step to take!
Do you have a petrol or a diesel?
Do you know if the replacement cat was a genuine one, and who fitted it? Did the new cat have a warranty?

Slightly guessing here but it could be the post-cat lambda (there are two, one in the exhaust manifold, pre-cat, and one after the cat).

Thanks for the help guys, i'll see if i can get any further info on the catalyst.
It's a 1.6FSI, i didn't realise there was another lambda sensor post cat? Chap at the garage said it was just an emissions check sensor. i'm tempted to buy another new cat and sensor (whatever it is post cat!) and hope for the best although i still don't understand how either of these being faulty could cause the car to drive SO badly?! Might i be barking up the wrong tree with the exhaust system/Cat...is there any chance given the fault code that it could be an air mass sensor or something? Thanks

Hi EmmyJ
Post cat just means the second sensor is positioned after the cat. It is in effect an emissions check sensor which measures oxygen it the exhaust gas. A leaky exhaust gasket or joint can allow air into the exhaust pipe which results in false readings. The procedure to check for leaks is to stick a rag in the tailpipe while the engine is running and listen for any leaks (don't fully seal the exhaust pipe or the engine will stop)

Cheers Spike

hi emmyj i'm sure that is the error code i had when my fsi had the eml come on. i had my independent guy vagcom the car and monitor the sensors---now this is just from memory so keep that in mind! he watched the lambda probe temp and it was under the 165 deg c so the ecu was putting the heating element on every 2 seconds and also adding air to the exhaust to try and raise the cat temp.
so the outcome was he said the first cat had failed so ordered a new one(audi part).he also pointed out that the second cat was broken inside and was rattling,so ordered the second cat also(audi part).since having these swaped it has been fine. my guy's opinion was the car prior to me owning it had done short trips (only 20k in 4 years) so had not allowed the cats to run up to proper operating temp.i went for genuine cats as i have no intension of selling my a2 and i do 20k per year.
i do hope this helps as a side note i was told the second cat was really overkill and unnecesary---if i had had the time i would have gutted it and put it back on empty.
good luck hope you fix it soon mikee

Hello. Last night a P0420 code popped up so there's something wrong with the exhaust somewhere. Would a Vagcom reading help pinpoint the fault?