Had that problem on my Landcruiser recently. Non-operating EGR. The EGR exists to reduce NOx emissions by lowering combustion temperatures. I troubleshot mine with a Mity Vac. What kind of car? Mine had a bad EGR valve, a bad modulator and a bad vacuum solenoid valve. You can pass emissions by forcing the EGR to be on all the time. I did that because the VSV which controls the on/off of a Landcruiser EGR had not arrived before I had to take my free follow up test.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13