B737-800
Flight Crew Air Transport Pilot (ATP)
Troubleshooting
Aircraft Equipment Problem Critical
Aircraft Equipment Problem Dissipated; General Declared Emergency
Aircraft
While on final approach I called for gear down; Flaps 15. The nose and right main gear came down and locked okay; but the left main did not extend.  We declared an emergency; consulted the QRH and cycled the gear twice with no change in results. Later; level at about 1;000 [feet] AGL for a fly by the Tower; we lowered the gear once again but this time we got three green lights.  The Tower took a look and said all looked good. We were then sequenced around for a landing during which we left the gear down and landed uneventfully. We stopped on the centerline while CFR took a look at the gear.  They all looked good so we taxied to the gate followed by the fire truck.
A B737-800 flight crew abandoned their approach when the left main gear failed to extend.  During a subsequent fly-by of the Tower the gear extended normally and an uneventful landing ensued.