A Mechanic reports an Engine Fire light illuminated on start-up; on a Eurocopter AS350 Ecureuil helicopter at a gate. He had previously disconnected two cannon plugs on the engine deck for access to an engine oil line that was rubbing on one of the engine cluster drain clamps.

Narrative

I was called back to work after shift to inspect an oil line that the pilot noticed was rubbing on one of the engine cluster drain clamps during his preflight. I disconnected the two cannon plugs on the left hand side of the engine deck for access to the drain clamps and to inspect the area of the oil line which was in contact with the clamp. When I reconnected the plugs; I thought that they were both connected and fully seated; I asked the Duty Pilot to check them both (which he did and agreed that they were secured).About 2 hours later he called me back in because; while starting the engine for a flight; the engine fire light illuminated and would not turn-off. I came back in and I removed both plugs [connectors] thinking that maybe I had accidentally bent one of the [connector] pins while re-connecting them earlier; what I found was that the large cannon plug had not fully seated even though it felt securely tightened. I cleaned and inspected both plugs for damage and finding none; reconnected them; being extra careful to get them both fully seated. When power was put to the aircraft buss; the fire light was out. Testing the Warning Light System indicated the light was functioning as designed.In the future; I will remember those plugs [connectors] require more than a few turns to re-connect and that they have indicator lines on them to ensure they are fully seated. I will inspect them more carefully after installation to ensure they are properly connected.