The GP are for one thing only: easy starts. So if your car starts easily after a short GP warm up and runs smoothly on all cylinders, that is the best test of GP you can get.

My one 300SD always started a little hard. Then I got another 300SD and I noticed the GP warm up time (indicator light on) took much longer but that car started easily. So the solution to getting the first car mentioned above to start easily is to wait about 10 seconds after the indiator light goes out to start the engine. Then it starts on the first crank or 2.

The GP system has a built in tester for the GP. If any of them go open, the indicator light will blink after the engine starts. If this dosesn't happen and your engine starts easily, forget about your GPs. They are not a maintenance item like spark plugs.

P E H