If the glow lamp is illuminating while driving, the problem has nothing to do with the glow plugs themselves. Replacing the glow plugs would accomplish nothing. (Believe it or not, the glow plugs are not "tested" periodically while driving.)

Most likely, a poor electrical connection (possibly the key switch) is causing the glow timer to initiate the glow sequence, just like happens when the key is moved to the glow position. A temporary loss of power will cause the timer to "think" that the key has moved from RUN to OFF and back to RUN.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.