94 E320 Failed Emissions Test

[vellosg1]

Results were:

High Speed Emission Results (25 mph):

HC Standard 120 Test Result 119 PASS (barely)
CO Standard 0.67 Test Result 0.06 PASS
CO2 Test Result 14.9
O2 Test Result 0.1
NOx Standard 848 Test Result 248 PASS
DILUTION Standard >6 Test Result 15.0 PASS

Low Speed Emission Results (15 mph):

HC Standard 124 Test Result 463 FAIL
CO Standard 0.69 Test Result 0.27 PASS
CO2 Test Result 14.7
O2 Test Result 0.4
NOx Standard 848 Test Result 1345 FAIL
DILUTION Standard >6 Test Result 15.0 PASS

Took the car to an independent, just across the state inspection. Result: "Checked EGR, OK. Oxygen sensor, OK. Recommend new Catalytic".

Took it to the MB dealer:
Results: "Catalytic OK, injectors OK. Recommend innitially to replace air injection check valve and MOT valve, remove intake manifold and replace EGR tube. Price: $900.00. However, final price can only be determined after performing first phase".

The car has 127,000 miles and had the 120,000 revision made at the MB dealer. Has new oxigen sensor, air flow meter, plugs, etc... No codes (single blink) and the only abnormality is a slightly fluctuation in idle speed (between 500 - 700 rpm).

Need someone to give me a clue of the probable causes and a sequence of tests I could perform to find them and get them fixed.