ransky,

Was the plug with high resistance the number one plug? If so, this is probably your whole trouble. The relay measures the changing resistance of the number one plug and uses this to determine when to turn off the dash light.

My experience in the past has been that if the glow plugs are a few seasons old, just replace all five. This is usually a miracle cure.

Also the resistance measurement does not tell the whole story with these plugs.

Good luck,