Make sure you unplug the (5 pin) connector from the relay to the GPs when you test the individual GPs. If not, you will not be measuring the actual resistance through each GP. I normally measure the resistance from the connector to ground for each GP, should be in the .5 to 1.0 ohm range (there are very small numbers on the plastic connector). Normally they will just read infinity (open circuit) when they fail. When you detect a bad GP, check again at the GP connector to make sure it's not a broken wire. I doubt that any carbon build-up will affect the electrical resistance.