The w124 om603 also tells you via the glow indicator if some plugs are bad using the glow indicator light on the dash. The GP relay monitors plug #1 and #6 individually and powers #2 through #5 as a group. When it detects and imbalance of power draw it does something different with the indicator light. I forget what.

But I find that I can tell well enough when a GP is bad: the car starts funny.

I can find which one in a few minutes: probe the GP relay harness going to plugs with an ohm meter.

No special reader tool required.

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Cheers!
Scott McPhee

1987 300D