Hi all,

Right..........

I keep getting pre cat o2 sensor codes come up (16804), this in turn is triggering random misfire codes (16685,16686,16687,16688).....I have changed the o2 sensor and still no changes....

MAF, crankshaft sensor are new, as are all of the coil packs and plugs. Had smoke test done on all vacume/auxilery pipes and all perfect.....

Would a slight blow on the downpipe/cat cause this to give misreading fault codes???

Your thoughts/opinions are very much welcome!!!!

Steve

this thread on vwvortex thread suggested by rosstech might be worth a read

yes a blow in the exhaust or downpipe can play havoc with 02 sensor readings especially if it's before the catalyst... when you go onto over-run when it's pulling a heavy vacuum it pulls in fresh air through the leak/holes and it tricks the 02 sensor into thinking it's running far too lean and subsequently tries to richen it up hence the missfires... oddly it's the 2nd lambda probe it uses to determine the efficiency of bank 1 (the cat and 02 sensor combined) it kind of uses the lamda reading from the 2nd probe (narrow band) to 'check' if the cat is working properly...

i've heard of a dodgy or misreading MAF throwing up this fault before too, the reason it doesn't show up as a fault code relating to the MAF is because the signal from it is still within the normal operating range of the MAF so the ecu just assumes it's working correctly... also don't discount the MAF as being faulty just because it's new especially if it was a pattern part or chinese ebay special.. i guess you could test this by examining the meausring blocks (channel 3 iirc but don't take my word for it)...

this thread on vwvortex thread suggested by rosstech might be worth a read

Cheers!!!!! That makes sense now!!!

yes a blow in the exhaust or downpipe can play havoc with 02 sensor readings especially if it's before the catalyst... when you go onto over-run when it's pulling a heavy vacuum it pulls in fresh air through the leak/holes and it tricks the 02 sensor into thinking it's running far too lean and subsequently tries to richen it up hence the missfires... oddly it's the 2nd lambda probe it uses to determine the efficiency of bank 1 (the cat and 02 sensor combined) it kind of uses the lamda reading from the 2nd probe (narrow band) to 'check' if the cat is working properly...

i've heard of a dodgy or misreading MAF throwing up this fault before too, the reason it doesn't show up as a fault code relating to the MAF is because the signal from it is still within the normal operating range of the MAF so the ecu just assumes it's working correctly... also don't discount the MAF as being faulty just because it's new especially if it was a pattern part or chinese ebay special.. i guess you could test this by examining the meausring blocks (channel 3 iirc but don't take my word for it)...

All sensors are genuine Bosch items, but I'm 99% sure after reading this it's the blow/leak in the exhaust which is causing the issue!

Time to get it off and check it all!

Thanks guys!!!!

Steve