Pulling the plugs and checking them is free. Pull them all out and use your ohm meter to test each one. I usually set mine for the lowest resistance setting and if i get any resistance on a plug I replace it.

Also if you have a spare battery sitting around you can test the to see if they get hot. Just don't hold on to it with your bare hands or you'll burn yourself, then drop the plug, and then it will be bad for sure. Give it juice for only a second or two to see if the glow red. If they don't then you need to replace it.

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Based on very recent experience, replace all 5 with new at the same time. Really. It's a pain, but will likely solve the problem.
I replaced the two bad ones, then within 2 days the other three died on me.
The box end ratcheting Gear Wrenches from Sears (8mm and 12mm) were a life saver. On the fifth one, definitely put a towel below to catch the nut, just in case. Once it goes down into that abyss...

Also, check for any moisture at the relay box. If it's not bone dry, something goes funny on it. I had one box where it got a little wet from being uncovered with some drizzle, then it failed. I let it dry out indoors and reinstalled and it's been fine, same test results you had.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

I replaced the one GP that had higher resistance.
My GP light came back on as it should.
Car starts almost instantly (like it did before).

$10 for Bosch at Autozone, normally stocked item.

I'd like to know what the part number you got from AutoZone. The price I saw for mine was $17.99. I have the pencil type. Are yours different?

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1984 300SD 148K Gold
1979 300SD 142K Cream
2001 Audi TT Roadster 60K
2005 Chevrolet 3500 13K

autozone part # 80006
9.99 bosch pencil type 81 and up 617 motor style glow plugs
bosch # 0 250 201 039
Made in France. not Germany though.
John
get two at a time, and get your autozone card punched or swiped whichever type card you have... 20.00 adds up quick!
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!