1999 Peugeot 406 voltage needed to open injector selenoid for hdi 406

Is your battery strong enough when cranking ? possibly " lazy " starter drawing it down ?

You know I don't actually know!
some of them are crazy high voltages.
normally if I'm testing them I'd use a current clamp and a 'scope.
Not voltages!
or a simple "Noid" lamp to check for switching.
But if you are getting erratic voltages the first suspect would have to be wiring.
I've seen a number of French cars with what can only be described as dust cored wire.
The internal wire cores crumble.
So very high resistance.
If you don't have access to a 'scope and amp clamp the a load test would work.
You need some jumper wires with alligator clamps and back probe needles for getting into the back of the ECU plug and a load,
I use a 21W indicator bulb.
Disconnect the injectors and the ECU,
12V from the battery through the load bulb into the back probe pin and into the ECU harness plug for ONE injector wire,
then on the other end of the injector wire a jump wire to ground,
better back probe with a pin so as to not distort the injector terminal.
Bulb should light on every wire tested with the same intensity,
If you have one a multi meter set to amps and in series should read the same amps.
Sounds fiddly but it's only 8 wires to test.
Any wires tested where the bulb glow intensity is low is suspect.
Similar tests on the ECU for power and grounds is helpful.
Forget testing anything with a multi meter.
you can get a full battery voltage flowing through ONE strand of wire.
Only by loading a circuit can you tell if it can actually carry current.
After all this if it's still doing it it's time to send off the ECU for testing.

what a faff....just get a multimeter and check the resistance of the wire...anything above an ohm is a resistance

I'd suggest that Paulione looks into a Set of leads called "load pro".
There's a lot on youtube.
Including the inventors fun and games suing Snapon for copyright infringement.
! ohm is a tiny resistance.
Guys name is Dan Sullivan and he makes his case.
And yes I own a set.

What is the voltage needed to trigger open the valve/solenoid for hdi injectors during firing.....?

about 80 volts

^ Probably got it fixed as the post is more than 2yrs old .

but that doesn't matter as its information for others

Good point , but with this forum not many use the search facility and tend to post afresh .

but if they do then they have the answer