Initially had a friend diagnose car at his shop & vag com indicated codes for misfires, o2, MAF sensors...suggested change of spark wires, spark plugs, and sensors. Thus far, MAF has been replaced & had the plugs & coils replaced from ignition coil recall at the dealer. The dealer also ran codes that indicated pre cat sensor & engine coolant sensor need replacing. Car still runs at a rough idle & will stall out even after coil & MAF replacement. I've found out that a Vacuum Hose leak may also cause o2 (pre cat) sensor to appear bad. The sensor is $97 ECStuning, $240 @dealer(yeah right!) $170 @audi wholesale & a universal o2 sensor @ Partsbin for $28. My question is how to check if i have a vacuum hose leak $12-$42 for replacement. I don't want to replace that don't need replacing. Which pre cat sensor to purchase? & how to install myself. Much Thanks!