1998 E300 Code P0243 Solved!
 

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!

Great job! Way to stick with it!

Dude, you're more stubborn than I am and that's saying a lot. Did the guy figure out what was plugging it up? Excellent drill down to the fix! You should get a medal.

wow, clogged converter?!
who woulda thunk it!

I've been researching this problem and that's the first i've heard of this
anyway, nice work.

rack up another clogged converter
 

The car has been on a steady diet of homemade 100% biod since '09. The guy at the exh shop said that the pluggage was as bad as he had ever seen (I did not get a chance to see it). Seems that when the fuel is not quite up to spec it can plug a converter.