Originally Posted by kerry

If one glow plug is out, none of them will work, not even the ones earlier in the series because the circuit is not complete. If you put one probe of the ohmeter at the firewall hot wire coming to the first plug and the other on the ground wire at the last plug at the front of the engine, you should have a complete circuit. If that circuit is open, there's either a bad plug or a disconnected wire.