These jeepforum threads are fantastic! My check engine light was on with my 2007 WK 3.0 CRD, 90K miles, and I'm no longer under warranty. I'm a shade tree mechanic with zero diesel experience, so I relied on the service department at the local dealership to identify the problem. This $TEALERSHIP told me that the OBDII system indicated that I needed 6 new glow plugs as well as a new glow plug module. Estimated bill would be $1,000. (Would my vehicle even start if all of the GPs were bad and the module failed?) Since there were no apparent performance issues and I was now suspicious of this $stealership, I declined the work, retrieved my WK and headed home.
On a whim, I stopped by Advanced Auto Parts for advice. The employee grabbed an OBDII scanner from her bench, plugged it in to my vehicle and advised me that my #5 glow plug was faulty. That's all. These GPs weren't in stock, but she was happy to order one for me to the tune of $33.
Looking through the jeepforum threads, I determined how ridiculously simple it would be to replace the GP myself. These threads walked me through the process of locating the appropriate cylinder, and gave step-by-step instructions as well as a couple of pitfalls to avoid. It took me a total of 20 minutes to replace my #5 Glow Plug (fortunately the most accessible), and if I had known what I was doing in advance, it would have probably only taken 5 minutes.
Harbor Freight Tools happened to have a simple CAN OBDII scanner on sale at this time (Item # 98568), so I picked one up for $55. I was able to confirm that the fault no longer existed, and this scanner also allowed me to reset the check engine light.
I saved $900, and I now have a simple scanner for future fault identification on multiple vehicles.
I also have absolute respect for www.jeepforums.com