Quattro not passing emissions, Tom Sidders

[John Larson]

Yup, the temp differential test is not only real, it's one of the
easiest diagnostic tools for determining the effectiveness of the
converter. Another test you can do, which requires an emissions tester,
is to sample the emissions at the little test port in the engine
compartment. The one with the blue cap. Check it there, check it at the
tail pipe, and compare. You might be interested to know that the primary
result of the gasses passing through the cat is a drastic reduction in
the HCs. While the CO might drop a bit, it won't be that much. High CO
is a function of basic mixture settings and the O2 sensor. HTH, John