It's more than likely you have 2 bad glow plugs. Mine stopped illuminating the light when 2 finally failed to glow any longer... I agree with Dave, replace all 6 and it is a major PITA with the intake on, but I've done it.

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1960 MB 190Db w/ full sunroof
1985 Toyota Pickup
2006 Honda Odyssey EXL R&N
2001 VW Jetta GLS TDI