John,

P0089 will set on that system for one of three reasons –
1 ECU sees rail pressure too low
2 ECU sees rail pressure too high.
3 ECU sees a fault with the circuit.

If the ECU cannot control the pressure and it sets P0089 then the ECU will shut it down.

Are you sure this doesn’t have a pump in the tank? The wiring diagram shows one (Opel TIS2K).

If you scope the reg duty v the pressure sensor signal while cranking then you should see the ecu commanding the pressure. That should give you a better idea of what is happening.
The reg current will vary with the duty cycle. Are you scopeing the current? Scope it versus the duty cycle and see if the changes in both match. This could show you a resistance change in the circuit.

What method did you use to check the HP pump?

Do you mean the duty cycle is 30% and not changing? As far as I recall that system will go into a default mode when it cannot control the pressure and that is probably why you are seeing the 30% duty cycle.

It might be an idea to check the pressure sensor as well since this is the source of the info which is causing the ECU to act up(unless the code refers to a circuit problem).

HTH

Alan