For 602's, the documented GP lamp codes are:

Fault indication by preglow indicator lamp

- Lamp fails to light up even during preglow,
lights up for approx.1 minute while dnving.
Fault: One or more glow plugs defective.

- Lamp fails to light up during preglowing and
also while driving.
Fault: Indicator lamp defective, line
interruption to the indicator lamp, preglow
time relay defective.

-Lamp lights up permanently.
Fault: Preglow time relay defective (relay
sticking).

- Lamp fails to light up, engine is hard to start
or fails to start altogether.
Fault: Short circuit on one or more glow
plugs, line interruption, preglow time relay
defective.

The way the system works is that the current draw of the plugs is checked by circuitry in the relay. The expected problem is that one or more plugs go open, and that will cause one of the standard signals documented in the FSM. I think it's possible for something odd, like a partially shorted plug, to cause an undocumented signal. So my suggestion is to begin by checking the resistance of each plug. Replace any plug that's far from the average reading. Recent new plugs mean nothing. These days, even new parts out of the box are suspect.