The people and information on this forum have been tremendously helpful to me, and I wanted to contribute this to help somebody avoid a bit of aggravation and expense. Here is how I got rid of my (stubborn!!!) P0243 code:

The car is a 1998 E300 Turbodiesel with 199,xxx miles. It started setting P0243 codes some months ago. I took the following steps to find the source of this code. I changed only one thing at a time and then tested to see if that cured the problem. It has been a rather long, drawn out affair!

Exchanged the turbo wastegate transducer and EGR transducer;
Checked vacuum source at turbo wastegate transducer (27” Hg);
Replaced vacuum hose from transducer to wastegate diaphragm;
Verified wastegate diaphragm integrity and action with Mighty-Vac pump;
Checked wastegate vacuum levels per Mercedes specs (Thanks, Terry Allison!);
Tested transducer vent filter with Mighty-Vac found no restriction;
Disconnected vent filter hose just to be sure;
Banged head on wall;
Checked all metal and rubber air piping from turbo to intake;
Replaced rubber hose to MAP sensor;
Added hose clamps to MAP sensor hose;
Verified MAP sensor intake hose nipple open and clean;
Graphed MAP sensor voltage vs pressure (vacuum and boost) – it was linear;
Banged head on wall;
Cleaned MAF sensor;
Replaced engine air filter;
R&R Intake manifold to replace large o-ring seal;
Pulled K40 relay board, re-soldered, re-installed;
Bypassed wastegate transducer – hooked vacuum source directly to wastegate diaphragm.

NONE of the above had any effect – the car just kept throwing the code and going into “limp mode”.

Monday, I took the car to an excellent muffler shop and asked them to check for an exhaust restriction. He drilled a 1/8” hole in front of the catalytic converter and attached a pressure gauge. One throttle blip told the whole story – the converter was stopped up. After repairing that problem, the code is gone and the power is all back!

Since I never saw any mention of an exhaust problem reference P0243, I hope that this information will help somebody. It was not at all obvious to me that a clogged converter could throw a P0243 code!

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David Learner
1998 E300
1998 E320