Date: 202108
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion; Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Ground Equipment Issue; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Contributing factors: ATC Equipment / Nav Facility / Buildings; Aircraft
Primary problem: Ambiguous

Narrative: During approach into ZZZ while receiving vectors was cleared to 9;500 feet MSL and told cleared for the ILS XX and to maintain 9;500 feet until established. Selected APP and aircraft captured GS and LOC and was descending on the app with GS and LOC locked on. Got a call from approach at 8;100 feet letting us know we were low and to climb up to 8;600 feet I immediately disconnected the auto pilot and initiated a climb while maintaining LOC. After crossing ZZZZZ I then recommenced my descent and flew visually using the PAPI as my guidance. Landed the aircraft within the touch down range and stopped on the runway. CA taxied aircraft to gate. During taxi we were informed by approach that the GS had issues and they thought it had been fixed. We initially intercepted the GS and LOC approximately 20 miles out. At approximately 2 miles out the GS was back in line with the PAPI.


Second narrative: We were doing the ILS Z Runway XX into ZZZ. Night time; good visibility; no clouds. ATC had us at 9;500 feet; turned us to intercept final outside of ZZZZZ and cleared us for the approach. Autopilot was on. We were flaps 1 about 190 knots at the time and about 22 miles on final. We intercepted the localizer and the glide slope at about the same time. At which point we were busy setting touch down zone; putting flaps down and gear. A moment later ATC gives us an altitude low warning; climb immediately to 8;600 feet. The flying pilot ( first officer) immediately disconnected auto pilot and initiated climb. We were around 8;100 feet; on the glide slope; auto pilot on when this was initiated from what we could tell. We had just passed ZZZZZ (FAF; but 17 miles out). The controller mentioned they had some glide slope problems just a week or two prior; and asked us if we were ok to proceed visually. Still being over 10 miles out; runway and PAPI in site; and glide slope still showing slightly below us; we proceeded visually with reference to PAPI and GS. The problem may have been from the auto pilot intercepting the LOC and GS at the same time and possibly overshooting the GS as it sometimes does when far out and possibly exasperated by configuration changes. This all happened about 15 to 20 miles from touchdown. Or it could have been from a faulty GS signal that they have had problems with previously.


Synopsis: Flight crew reported descending below minimum altitude on approach and climbed back to correct altitude. ATC advised flight crew that there had previously been problems with the glide slope.