The first time I had such a problem, I didn't think to disconnect the 5 prong plug at the relay box while I went probing the tips of the glow plugs themselves, so I got uniformly good readings, but a few minutes later I gave myself a big dope-slap and went back and rechecked with the connector disconnected -- then I found one plug with infinite resistance and one very bad, along with three good ones. I had realized that there has to be a point where those five circuits come together, so if the connector is left in place, electricity will take the path of least resistance and give good readings as long as there is even one plug which is ok.