B737-700
Flight Crew Last 90 Days 178; Flight Crew Type 10000
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
Cleared initially 31R into ZZZ when changed frequency and changed approach Runway to 31L. FO set 1200 FT in altitude and verbalized; but I did not catch because I was verifying frequency and altitude changes from new runway assignment. FO stated we looked low and started to climb. At 1800 FT I looked up at altitude window and stated we want 2200 FT not 1200 FT and simultaneously; ATC told us to climb. We were visual with the field and buildings climbed to 2200 FT and intercepted the glideslope and continued uneventfully on a normal approach and landing. [I should] verify any change on MCP with Pilot Flying before [going] heads down to make changes or don't make the change.
We were cleared initially 31R into ZZZ when we changed frequencies and changed approach to Runway 31L. I set 1200 FT in the MCP and verbalized it in error. The PM was verifying new frequencies and altitudes for the runway change and misses the incorrect input. I perceived we were low and started a climb at approximately 1800 FT in VMC. The PM realized the incorrect altitude set in the MCP and said we want 2200 FT not 1200 FT.
B737-700 flight crew reported descending below charted altitude on approach when the pilot not flying failed to verify MCP altitude change.