Glow plug life
A failure of one glow plug does NOT shorten/affect the life of other glow plugs; that is an unsurprising myth instigated by repair shops to make more money off naïve customers. There is, however, a line of reasoning on how you may save some money over the long term by wholesale replacement of all of them when one fails (read on), depending upon WHEN the first one fails AND it's money out of your pocket (ie, not covered under warranty) AND you don't want to try DIY replacement.
I just had the 3rd glow plug on my 2008 Grand Cherokee 3.0L V-6 diesel replaced yesterday after 90,000mi. The first one failed in 2010 at about 40,000mi. I grew up since the '60s with diesel cars (my father was a master diesel mechanic) and I don't recall witnessing such a short life on glow plugs before, but then these new ones heat up instantly whereas the old ones back then took nearly one minute before you could start the engine...ie, the heating stress is significantly more these days which may account for shorter life.
Remember, the purpose of a glow plug is to enable the diesel to burn when starting an engine that has not heated up to operating temperature yet. When you turn the ignition on, electric power is applied to the glow plugs and they quickly heat up to a cherry-red glow inside the cylinders, so that when the starter engages, the crankshaft begins turning and the fuel injectors squirt diesel into the cylinder, the fuel will begin burning to get the engine running (ie, piston compression is insufficient alone and needs that extra residual heat in the cylinder). This is particularly important when using biodiesel blends (biodiesel has a higher flash point compared to petrodiesel). After the engine has warmed up, the residual internal cylinder temperature is high enough to where the glow plugs are no longer needed, so they're turned off by the engine computer.
In order to get an extremely fast heat-up time for today's engines (consumer demand; it's annoying to have to wait 30-60 secs before cranking the engine), the glow plug's internal heating element must have a low resistance, thus allowing a lot of current to flow to produce more heat power (P=I*E, Ohm's law). This high power in the element eventually erodes it, and when it fails the element develops what amounts to a crack in it from the loss of resistive alloy, producing what EEs call an "open circuit".
Since my first one failed at 40K mi, second at 70K mi and third at 90K mi, I'll throw out the first as an anomaly, and average the second two to estimate average life expectancy at 80K mi. So, I'm expecting the remaining 3 in my engine to fail anytime now. IF you use that reasoning as a guide AND you don't do the plug replacement yourself each time, THEN you may want to consider the financial trade-off of getting all of them replaced by a repair shop IF the first one goes after 70K AND it's significantly less expensive for shop labor to just do the rest of them so that you essentially reset the clock. After all, glow plugs WILL ALL eventually fail at some point in the future...there's not avoiding it as the nature of the beast of how they work makes it inevitable. (Same is true for injectors, BTW, albeit for different reasons.)
The engine computer is designed to monitor the glow plugs for this glow plug open circuit condition, and when it occurs to one or more of them the "check engine" indicator lights up on your dash. You can easily confirm the problem if you have a "code scanner" to plug into the Car Area Network port under your dash, reading the code and doing a Bing/Google search for translation of the code. Sometimes, I've noticed a slight hesitation in starting my Jeep when a glow plug failure has occurred, esp if the outdoor temperature is below 40 deg.
FYI: I stumbled across this thread today while searching online for replacement glow plug parts. I used to get mine replaced under warranty, but that has now expired so it cost me $400 yesterday for that job. Since replacing a glow plug is not a lot more difficult than a spark plug, I'm gonna do it myself next time and have already found replacement plugs for my model for about $20 (Standard, Bosch, Beru make them).
What if the Hokey-Pokey IS what it's all about ?