PEH is correct. I'm sure you only had one bad glow plug.

I broke a key rule this fall and replaced all of mine as preventative maintenance. (rule broken, if it ain't broke, don't fix it)

As the winter wore on, I had two of the new ones go bad,,,,all were Bosch. I later found that the fuse was bypassed, replaced the fuse and have not had an issue since (not sure if this led to the bad new plugs or not)

Bottom line, a bad glow plug is easy to diagnose and replace. I now see no real need to replace them unless you know they are bad. My problems started when I replaced the original 26 year old GP's. NEVER AGAIN.

JCD

PS, the mechanic should have replaced the GP first. If the problem was solved, then he should have left the injectors alone. I'm still a relative newby, but I can't make the connection between bad fuel, injectors and glow plugs.