99 A4 1.8T Quattro w/ 125,000 miles. Recently my check engine light came on and my bottom-of-the-line scanner tells me I have engine codes 16518, 17536, and 17544 resident. Car idles noticeably rougher as well. Anyone have experience with these codes? I did an internet and Audiforum search, but got mixed advice. Right before I got this light and these codes I checked my K&N air filter, but I made sure it was all buttoned up tight and there are no vacuum hoses loose or broken. Really appreciate any help.
In the last year I've replaced the timing belt, clutch, brakes, windshield, abs controller, and a whole host of other items my wife cannot name but can recall the prices of! No way I'm letting this one die on the table now.....

Thanks

 

I took apart the air box and cleaned the MAF. Runs great, but the check engine light came back on. Now there's only one code: 16518. I believe this is the O2 sensor? Does anyone know if this can be anything besides the O2 sensor and which O2 sensor I need to replace (first or second one in the exhaust line).
Thanks!