[BNaylor]

There are 2 tests that can be run for a possible clogged CAT convertor or excessive exhaust backpressure or restriction. A pressure test where a special adapter/fixture is connected a the front 02 sensor bung. The reading should not exceed 1.25 psi. Muffler shops or the GM dealer can perform this test.

The easier DIY test is to procure an automotive vacuum gauge (0-30hg). You tee into any vaccum line at the intake manifold. At idle with engine properly warmed up the reading on the gauge will be in a range of 15-22 in hg. The rpms are increased to 2000 rpm. The gauge will fluctutate initially but then maintain a steady reading of similar to above but possibly a little higher. If the reading is erratic or starts dropping off towards the low range 15 psi or less then there is a high probability the CAT is clogged.

Low readings at idle (less than 15) would indicate a really bad clogged CAT or other problems like intake manifold gaskets.