86560SEL, if a lamp is burnt out your W140 should have a "Lamp Defective" indicator on the instrument panel. I'm not familiar with that particular automobile ... so either a lit warning light, or the words "Lamp Defective" on the panel. Otherwise, the problem is elsewhere. It's likely something other than a lamp problem because the turn signal would flash faster with only a lamp burnt out.

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Fred Hoelzle