One headlight not working on full beam (bulb is ok)

[umog]

Headlights both working fine when on dip (on Porsche 944). However, when switching to full beam only one bulb lights and the other goes out. I have tested both bulbs in another car and they work fine. I have tested the bulb holder and appear to be getting 12v to the terminals.. Any ideas what to look for first?

[LeoSayer]

Have you tried using another bulb in the faulty unit?

[NDA]

Have you cleaned up the fitting that the bulb goes into?

Is there likely to be a fuse for dip and a fuse for full?

[umog]

LeoSayer said:

Have you tried using another bulb in the faulty unit?

Yes - problem persists

[umog]

NDA said:

Have you cleaned up the fitting that the bulb goes into?

Is there likely to be a fuse for dip and a fuse for full?

Ive given it a light brush but thought that because it works on dipped the earth connection must be ok
Yes there is a fuse for both - again these are both fine

[adeel_gt]

Is it a H4 bulb? If so ut will have 3 terminals one for the earth, one 12v for dip beam and one 12v for main beam have you checked them both for power with the switch in each position?

[Penelope Stopit]

Check if the fuse for that filament is live when the headlights are switched on, if it is live there is very likely a break in the cable somewhere between the fuse and the bulb, breaks in cable are often found to be in the engine bay loom