Date: 201702
Local time of day: 1201-1800
Location: SCT.TRACON, CA
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B X
ATC / advisory: TRACON SCT
Aircraft component: FMS/FMC
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Second reporter function: Pilot Not Flying; Captain
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Contributing factors: Company Policy
Primary problem: Company Policy

Narrative: On arrival into LAX; before beginning the actual STAR we completed the briefings for the STAR and approach (which we had to make an assumption about and chose to brief 24R but also back up with 25L) so that we could complete our Descent Checklist. At this time the FMC was then loaded with 24R. During the descent on the STAR; after given the clearance by LAX Center to descend via we were cleared for the approach to 25L. Upon hearing this we changed our frequencies to 25L and minimums altitudes in our baro settings and both reviewed the chart for numbers for 25L verifying they were correctly entered. What we forgot to do was change the approach in the FMC which would change the transition after the RIIVR fix on the arrival. Shortly after passing RIIVR; I noticed the NAV tune disagree message and realized what we had done. As I quickly tried to fix what was in the FMC; ATC queried if we were lined up for 25L to which we responded we were fixing our course now. We were then on course to join the next fix on 25L which was KRAIN. We were still probably at least 3 to 4 miles outside KRAIN. ATC then made us stop the descent at 10;000' and make a 360 degree turn to rejoin the final for 25L and recleared us for the approach. Once established on that approach they switched us to 24R Approach where we made a normal landing.


Second narrative: Company briefing procedures required the guessing of which runway; approach and transition to expect. A best guess was made and briefed; based on personal experience flying to LAX. During the busiest part of the flight; at or below FL180; the briefed runway was changed. Interestingly; at the last minute; it was changed YET AGAIN; to the originally expected runway that was briefed; though this was not part of the event. There are many things that need doing when there is a runway change on arrival to LAX; and one was left out; the selection of the new runway; along with its transition segment on the STAR. Everything else was done; the selection and verification of the correct frequency; verification of the correct course and touchdown zone elevation; a quick brief on which taxiway to expect turnoff and completely different autobrake consideration. The missing of the new runway and transition caused the aircraft to remain on the originally briefed transition. This was caught simultaneously by the PM and PF shortly after the aircraft began on a course that did not align with the revised one. It was also caught by ATC as well; who provided vectors back to course.


Synopsis: B737 crew reported a course deviation on the STAR to LAX when runway assigned was not the runway briefed and programmed into the FMC.