Do you have an OBD2 code reader that can display live sensor data? Set it to live data and have it record the situation where the code is triggered if you can replicate it. Since P0400 is triggered by a self diagnostic test which is dependent on several inputs (TPS, MAP, EGR lift RPM, coolant TEMP) I'm guessing one of these sensors has an intermitant fault.

What model year is your E300? There were a lot of changes between 96-97-98.

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