A person on another forum got a Cosmo lamp streetlight, a new fixture with a used bulb, and reports the following:
It worked for 5 minutes when he tested it out. He put it away, and the next time he plugged it in it acted like it was trying to start, but never got beyond the beginning of the warm-up.

Bad bulb? DOA ballast? Something else? Could it be that the 240 volt mains was just low enough that it kind of works sometimes?

I had a case where i rescued a HPS 250W lantern. Choke was dead (destroyed by rust over many years of moisture and no use), ignitor was ok. I put in 150W choke and retrofitted it to 150W. It worked perfectly few times, then ignitor died for good. I replaced the ignitor and it works ok ever since

But in my case the ignitor just did not do anything - connect power, no starting voltge, no flashing or any light at all from lamp



Here if the CDM lamp is firing i guess ignitor is ok. Choke is probably ok too

Check connections :
Tighten what he can and try again

Remove ignitor, connect a 240V GLS lamp to the output, power up. Is it lighting up stable ? No flickering when messing with the wires ?

Cheak isolation breakdown parallel to the lamp :
Connect ignitor, remove lamp, power up. Are there sounds of arcing somewhere ?

If nothing else, try to swap components

Choke and ignitor can be tested with a simple HPS or MH lamp of same wattage, no need to buy a CDM lamp for a test (for HPS lamp, one size up or down is ok for a test too - Nothing will happen to it)

@Ash: Cosmo CMH lamps operates only on a dedicated electronic ballast.