I have a 1993 300E (M104 motor) that is almost 6 months out of registration... and I'm still trying to get the F****** to pass! I had the head gasket replaced so I have new plugs and wires... all my vacuum leaks were fixed, new 02 sensor, tried the PASS SMOG "snake oil?"... have a new EGR valve and am waiting for delivery of my EGR exhaust return line because the existing one is plugged (but I heard the EGR has nothing to do with HC's, which were high on my car)... so hopefully the EGR solenoid is still good...
I have new tires, brakes, water pump, top end... it's a great car... but SH**!!!
So no noise at all from the CAT pretty much means that's not the culprit...
WHAT gives???
What do you think helped you pass smog?
THanks!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

Hi David,
it can and does get frustrating without the resources needed to throw at the car.
High HC, Hydro Carbon, meaning there is too much Hydrogen and Carbon in the exhaust.
Unfortunately, in my and only my opinion, this points to a bad CAT. The CAT is supposed to take out the Hydro Carbons or at least reduce them to a state required threshold.
Unless, there is a leak in your Cyl Head and somehow adds coolant to the air fuel mixture, I would point my finger at the CAT.

The EGR in general simply takes some of the exhaust back to the intake, to re-burn what has exited unburned. The Valve it self operates on vacuum and activates at engine RPM's higher than idle (probably starting at low load).
To test it, you can apply some vacuum and the membrane should open the valve.
There is a SOV (Switch Over Valve) that is controlled by the ECU to apply the vacuum. Applying +12V will switch it on and off as you apply the voltage and take it off. Disconnect the control wires and apply the voltage source. Polarity shouldn't matter! You should hear a clicking sound. A 9V block battery might work??

The feedback pipe is notorious to be blocked, which really makes sense to me. The pipe is actually quite long, so here comes all the hot gases from the exhaust and travel all around the engine block until they arrive at the relatively cold intake manifold and adheres to the pipe walls. Resulting in a blockage over time. In many cases the pipe can be cleared with a non abrasive brush, some engine degreaser and compressed air. Objective is to keep the inside smooth otherwise the colder fumes can better adhere to the scratched pipe and you'll soon do the same thing again.
I would first see and do a smog pre-test after the pipe either has been replaced or cleaned out. It might get your HC down, since some of the exhaust is recycled.
If that doesn't work, my guess would be the cat.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert