Sounds like they're guessing & not testing. Of course the first thing to check for is a bad engine wiring harness. Look for insulation falling from the wiring. When the computer checks the EGR system it looks for vacuum drop in intake when the valve is energized. Make sure that it is tested. There are about 10 different O2 sensor codes, so what exactly was the code?? It should start P0 ??? or P1 ???.

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MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 16V ITS SCCA racecar