Your relay self-test feature is telling you that one or more glow plugs have failed.

You can do a quick check with a meter at the glow plug relay. Black box on the forward edge of the left front wheel well. Be careful under the cover, the big screw head and strip fuse have direct battery voltage on them so watch out you don't short that out.

There is a 4-pin (control) and 6-pin (to glow plug) harness. Unplug the 6-pin plug and check resistance of each glow plug to the car body. Meter pin 1 to frame, pin 2 to frame, etc. You have a 5 cylinder engine so pin 6 is empty.

If you have one or more plugs where the resistance is > 1 ohm then there's your problem.

Glow plug change on an OM602 is not too difficult. I would go ahead and change all 5 while you are in there.

The intake manifold has to come off unless you are a contortionist. So be sure to also order the intake manifold gasket, and cross over pipe gaskets as well (o-ring on one end, flat gasket on the other).

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech