P0400 Exhaust Gas Recirculation Flow Malfunction will have to be repaired and the trouble code cleared before your 1996 Mercedes S320 can complete the drive cycle and set the Catalyst Readiness monitor.
With that said, having replaced the EGR valve alone may have not corrected the P0400 due to a few things. One, you want to make sure manifold vacuum is getting to the EGR valve in order to open the valve as needed. Second, you'll need to make sure the silver tube leading to the EGR valve from the exhaust manifold is free and clear of obstructions (typically carbon build up), and third, you'll need to make sure the vacuum line from the EGR valve to the EGR pressure sensor is not broken, disconnected or brittle. Any one of these three faults will trigger a P0400 trouble code.
Using a hand-vacuum pump check that your Mercedes will open the EGR valve while the engine is idling. Once the valve has opened you should notice a significant drop in engine RPM. If the valve opens (which it should since it's new) yet you see no RPM drop, you should check to make sure the pipe leading to the EGR is not plugged up. If there is RPM drop, then we can assuming that no restrictions exist and the fault may be that the EGR valve is not receiving manifold vacuum, either because a vacuum line is broken or disconnected, or that there is a vacuum line obstruction somewhere.
Check the vacuum line leading to the ERG valve using a vacuum gauge. While the engine is idling, snap the throttle. You should notice manifold vacuum on the gauge while the throttle is off idle. If this system too is functioning properly, the next inspection will be that off the EGR pressure sensor.
The sensor is located towards the front of the engine and connected to the ERG valve via a vacuum hose. This vacuum hose travels very close to the engine's manifold and gets very hot. It is not unlikely for it to become worn and brittle closer to the engine side. Check for its quality and hopefully you will have found the fault causing your Mercedes Benz to trigger EGR Flow Malfunction Fault P0400.
