Glow plug

You can test the GP by putting control light between + what you can find in fuse box and GP cable.
MB sedans do have plugs on the relay where after pulling it out you will find # of the plug for testing. I assume Sprinter has the same system.
Getting can of MAF or electronic cleaner and clean the plugs would be in order regardless.

I used to test my 6.2 glow plugs by spitting on a finger then RAPIDLY tap the glow plug right after the glow plugs cycled. Be quick or suffer a burned finger.

Smart folks, use an inductive ammeter and test amperage BEFORE there's a problem. Any and or all problems will instantly show up if you suspect somethings wrong.

Glow plugs do go bad. Replace it and be done with it.

If the glow plug is accessible you can use a test light, AMP meter or OHM meter to test it.
If using a test light hook up the ground end to a 12 volt power source. Touch the point end of the light to the tip of the glow plug. If the light lights you have a ok glow plug, if the light does not light up you have an open glow plug, not working. Glow plugs use resistance to work so this method will not tell you how good the plug is just that it is still working.
You can also use an OHM meter to test, hook up the ground side to ground and touch the plug with the positive end. Measure the resistance of the plug and this, if you have the specs for the plug, will tell you the health of the plug.
If you have a AMP meter you can hook the meter in line with the plug and it will tell you the amperage the plug is pulling. Again if you have the plug specs you will know the condition of the plug. Many plugs pull somewhere between 6 to 15 AMPs, just depends on the maker of the plug.
Sorry, forgot to add one important item. If you have a plug go bad, the plug can swell up and get jammed up in the threaded hole. If you try to force it out you can ruin the threads or break off the tip of the plug causing it to fall into the cylinder. If you do replace the plug and it comes partway out nice and then gets real tight do not force it, screw it back in and take it to someone that has the right tools. Do not want to have to replace a head or block.
Be careful.

fiend of mine just had 2 plugs go bad.Busted off or runded the head? $5000 repair. Make do without it. It'll start

Has anyone had this problem? Lots of Sprinter owners! The glow plug failures are so common that some are looking at them as disposable wear items. I have a failed #6 right now, with parts on the way. #6, driver side rear, is the one I hear about most often.

Bad diesel? Probably not, but not enough info to really know.

What would it take to fix? A new glow plug, obviously. A Bosch replacement can be had for $20. From the dealer about $50. Even though it is usually a quick and easy job, I've heard of ~$200 for labor because they are sometimes seized in the head with a risk of breaking off. The driver side glow plugs are more difficult to get to because of turbocharger plumbing. Some owners have them all replaced when one goes out. I just read a report of $700 for all 6 at a Mercedes dealer, including a new control module. I'm a little perplexed by the control module part. While in the Sprinter I5 a shorted plug would also take out the control module, I though that situation was fixed with the V6. Maybe not?

DO NOT go without replacing the glow plug. Even though the engine will start and run fine, DPF regeneration will not take place. A clogged DPF can cost really big bucks. If you're tempted to do the job yourself, I suggest you spend time reading about in on the Sprinter Forum.

You can read horror story on almost everything.
I was replacing GP on 1998 Mercedes diesel at 280,000 miles and all of them come out easy, when others reported broken plugs.
I check the problem to be mostly related to extended plug, that has sealing taper deep into the hole. When taper fails to seal, the extension gots glued to the head with sot and turning without torque wrench can break it.
The deal is to find breaking torque (available on the net) and set the wrench 10-20% less. If the plug is frozen, start soaking it with PBlaster for couple of days and retry.

Used OEM five times in 6 years with a 5.2

Changed to Delco and the flaky lifespan vanished.

If you can, loosen the glowplug a little...

Dribble Chevron TECHRON onto the glowplug.

Wait overnight.

Polyurea destroys the lattice bonding of carbon. Chemistry 101

A glow plug is an electric heat element.. In many ways just like the electric element in your RV Fridge or water heater or a space heater.. As you may know all of these fail from time to time.. I've even gotten a brand new space heater with a failed heat element.

Thus.. Replace it is the recommended procedure.

Update: After getting a big runaround and now answers from Dodge dealer that said I had to go to M_B in Riverside, and Winnebago who referre me to my dealer, I got an answer from M-B. The glow plugs are warranteed for 8 years or 80,000 miles, whichever comes first. Mine is a 2007 chassis, so no go for me.

I got a quote from a local diesel shop for $150 parts and labor to replace it.