Catalyst is the first "muffler" in the pipe under the car. Non servicable, very expensive.

Make sure the pipe going into the manifold from the EGR is completely clean, and that the EGR valve itself is clean -- doesn't take much to plug it up internally.

I would find the problem with the boost from the turbocharger -- I believe (could be wrong) that it isn't particularly simple, but at the very least I would check the operation of the vacuum actuators for the boost controller and the EGR, as well as actual operation of the valves -- EGR must move, both open and closed, upon applying and releasing vac, and the wastegate control for the turbo must move as well. I don't knon what kind of wastegate control you have, it may not be obvious in operation.

Once you get the boost restored, recheck the ALDA.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!