Test them first. Disconnect the plug at the GP relay, use a multimeter, measure ohms to chassis ground of each plug.

GOOD = 1.1 ohms down to 0.8 ohms or so.
BAD = open (infinity, flashing digits, etc.) or 0.0 ohms (shorted internally)

Note which GP(s) are bad, should be two or more for lamp not to light.

Numbers on plug, and diagram on relay box, will tell you which cylinder number goes with which wire.

If they all test GOOD, check sockets and corresponding pins on relay box for corrision, clean it all off, and see if lamp now lights. I had corrosion on 2 pins, GPs were GOOD, corrosion blocked current so GP could not work. Cleaned the pins, and all is well.



Remove and replace GPs confirmed as BAD.

10mm wrench or socket to remove the wire from the GP. On OM603 the nut is captive on the wire, others be prepared for nut to leave the wire and hide...

An assortment extensions, and universal joints, etc. will allow a 12mm deep socket on the plug for removal like a spark plug. I like 1/4" drive, easier to thread through the obstacles.

A special reamer can/should be used to chase the carbon from the threads, but is not be absolutely necessary. Anti-seize on new GP threads before reinstalling won't hurt, either.

Install, 'medium' hand-tight, 'snug' torque. Replace wire, snug.

There are a lot of other threads on testing and replacing, may be harder to find. Others will post links soon, I bet.

Best Regards,
Jim