1999 E300 Glow Plug Confussion

[nate300d]

For background I replaced all of the GP's in the summer of 2005. At that time I did not check to see which or how many were bad. I just replaced all of them. When the GP light came on the Check Engine also came on. Once the GP's were replaced the GP light went out and I had the Check Engine light cleared out.

Last November a GP went out. This time there was no GP light or Check Engine light, but a cold started resulted in a rough running engine for a few seconds. This time I checked the resistance at the GP relay connector and found which one was bad. Only one was bad...replace and no problems.

Now today I start (very quick start) and I have a rough running for a few seconds at a cold start and both the GP light and Check Engine light are on. I checked the resistance at the GP relay connector and only two show any resistance; the other four are 0.0. I find that difficult to believe because I would think that the engine would have been much more difficult to start if four GP's were out. Also, why and I getting a GP light and Check Engine light? Is the GP light controlled by the resistance of only one GP? What is the parameters for the GP's that triggers the Check Engine light?

By the way the engine starts I would think that only one GP is bad (maybe two) and that is what I want to replace. Any wisdom on this??